SECTION 10
Colour exists on a map or graphic for the purpose of communication. The use of colour is of special relevance to marine and navigational charts, topographic maps and thematic maps whose primary purpose is to create a mental image of some of the characteristics of the region.
Communication in colour is more effective if the colours used are appropriate. Individual colours often have widely different cultural connotations with which the cartographer must be familiar. The colour blue, for example, can symbolize coolness, wetness, truth, constancy, loyalty, wisdom and despondency; in China it is the colour attributed to the dead. Similarly, red, which is an emotion-compelling colour, can symbolize heat, love, valour, energy, fire, cruelty, danger, wrath and sin; in China it is the colour of the living.
Colour is both used and abused widely. The psychological aspects of colour are commonly exploited in the fields of advertising and propaganda. A careful study of successful local advertising techniques in the cultural “target market” will often result in an appreciation of appropriate colour responses.
Hue refers to the specific wavelength zones of the electromagnetic spectrum and is the unique quality of a colour referred to by name, e.g., blue, greenish-blue, etc. Most natural and man-made colours are composed of combinations of wavelengths which approximate the spectral hues of a rainbow as seen when white light passes through a prism and is split into its component parts.
This refers to the relative lightness or darkness of a colour and is a measurement of the extent to which the colour reflects light. Thus brown and red are dark colours in comparison to yellow which is light. Value is considered to be the most significant aspect of colour, because it is a primary factor in the recognition of graphic variations. In the absence of colour, value ranges from white to black through intensifying shades of grey. Untrained eyes can readily perceive five steps in value from white to black.
This refers to the strength or fullness of a colour in comparison to a neutral grey, as described by the terms “brilliant” blue or “dull” green. A spectrally-pure colour is fully saturated; if the colour is diluted by the presence of other wavelengths of light as happens when it is screen tinted onto white paper, a desaturated hue results. Thus pink, which is created by screen tinting red, can be thought of as a desaturated red.
The three basic characteristics of colour discussed above do not occur separately.
value is the critical dimension of colour from the point of view of perceptibility. In contrast, hues arouse emotions or reactions, the most obvious being the warm-cool connotations already mentioned.
Intensity would seem to be the least significant of the three characteristics of colour, but it is a useful cartographic tool. The ability to differentiate between different saturations of the same hue is strongly affected by the area of the images and the spatial separation of the units. Adjacent legend blocks having the same hue with varying saturations can be easily identified. The differences, however, will not be as apparent if the coloured areas are widely separated on the graphic.
Against the normally complex background of a typical map with its varied symbols, text and area tints, fine lines of different colour appear identical. Fine discriminations in hue, saturation and value are only possible if there are no other distractions such as adjacent dominant colours. A fine line is the cartographic symbol most difficult to differentiate by colour, because it approaches the limits of perception. Differentiation by hue alone is the most common cartographic error in colour usage. Many different hues have visually the same darkness or lightness value and are therefore hard to differentiate. This is particularly true of fully saturated intense colours. Contrast effects are greatly improved when pastel shades or desaturated hues are employed for larger background areas. Thus if differentiation of fine lines by colour is necessary a second variable such as line width will ensure a clearer distinction. The same considerations apply to the use of small point symbols. Graphic redundancy again makes symbol differentiation clear (refer to Section 9.5). Maximum clarity occurs when hue, value and intensity are all manipulated deliberately to accentuate the vital aspects of the graphic and to subdue the related or background material.
Most coloured maps will be reproduced by printing on an offset or similar press (refer to Section 12), using inks which deposit a pigment onto the paper. The colour or hue of the selected ink is fully saturated when printed as a solid colour; desaturation of the solid colour, resulting in a colour tint, can be achieved by the use of photomechanical screen tints. Screen tints are reproduced on film or glass and consist of highly precise, closely spaced dots of a given size arranged in a rectangular pattern. The dot spacing on a particular tint screen is identified by the number of lines of dots per inch. A 65 line screen tint having relatively few large dots is considered coarse in comparison to a fine 150 line screen tint having many small dots. A fine screen tint will produce an even tonal effect in contrast to a coarse screen tint.
As the reflectance of light from a white surface is higher than from any coloured surface, a screen tint will increase lightness and decrease saturation. Screen tints are identified by the tone resulting from their use. A 10% screen tint will produce a light tone with only 10% of the surface area being covered with ink. Conversely an 80% screen tint will produce a strong, dark colour or tone. Usually screen tints are available in increments of 10% giving a large number of gradations of a single colour on one plate (Figure 10.1). Recent developments in the graphic arts industry are computer-controlled laser platemakers which are capable of producing any desired size and density of dot. This gives complete control over colour saturation and makes possible subtle gradations and colour mixtures very difficult to obtain by conventional methods. The cartographic potential of these machines is considerable.
The use of screen tints can result in considerable cost savings in colour printing. By using only two coloured inks and combining them using screen tints, a range of several colour variations may be achieved. Even if a number of these colour combinations or individual tints is discarded as not visually distinct, two coloured inks have considerable potential for the economical and attractive display of information. Many graphic products in the field of marine resources could easily be produced with such a simple combination as black and blue. Printing one or two coloured inks onto coloured paper is another economical solution.
In contrast to screen tints, which are used to represent selected areas in uniform grey or coloured tones, halftone screens are used to represent continuously changing ranges of tones or “continuous-tones” such as those found in hill shading or aerial photographs. Halftone screens are introduced in the photomechanical process because the printing process cannot easily reproduce continuous-tones. In contrast to screen tints which reproduce dots of consistent size, halftone screens produce dots which vary in size depending on the amount of light that reaches the film. The darker areas on the original reflect little or no light and produce large clear dots. The lighter areas reflect more light and produce smaller clear dots. This results in a negative of the original composed of dots of varying sizes.
Light is the small zone of the electromagnetic spectrum which is visible to our eyes (Figure 10.2). This zone is measured in wavelengths ranging from approximately 0.4 to 0.7 micrometers (one thousandth of a millimetre or one millionth of a metre). If a beam of white light is passed through a prism, the different amount of refraction of the various wavelengths causes it to split into its component parts. The same effect occurs when light passes through rain creating the rainbow. In the case of the prism we refer to the series of different spectral hues or colours displayed which the human eye can discern as the visible spectrum.
The shorter, higher energy wavelengths are the violet-blues near the 0.4 micrometre end of the spectrum. The longer, lower energy, wavelengths are the reds near the 0.7 micrometre region of the spectrum. The order of the spectral hues, known as spectral progression is violet, blue, blue-green, green, yellow-green, yellow, orange, and red in decreasing energy levels or lengthening wavelengths. This sequence, being a natural and familiar one to most people, is a logical choice when colours must be arranged in an ascending or descending manner.
Figure 10.1 Examples of screen tints expressed in 10% increments. (International Cartographic Association, 1984)
Figure 10.2 Electromagnetic spectrum. (After D.P. Paine, 1981)
Examination of the human eye has shown that it is most sensitive to coloured light at wavelengths of 0.55 micrometres, that is the yellow-green zone which occurs in the centre of the visible spectrum. The sensitivity drops off rapidly on both sides of this point, the lowest sensitivity being at the violet and red extremes of the spectrum. The perceived lightness or darkness of various colours is a measure of our visual sensitivity to the light received. Thus violet-blue and red are perceived as dark because of our relative visual insensitivity to these wavelengths. For example, in many regions of the world emergency fire-perceived as dark because of our relative visual sensitivity to these fighting vehicles were originally painted intense red, the most emotion provoking colour. In recent years, however, most of these vehicles have been repainted a yellow-green colour which has proved to be highly visible against the dark background of our cities. In some areas the accident rate of these vehicles dropped dramatically after the colour change.
For any pair of colours, maximum contrast occurs when hues of varied value are selected to maximize this effect. The following list ranks contrasting colours in a decreasing order:
Black on Yellow (most contrast)
Green on White
Blue on White
White on Blue
Black on White
Yellow on Black
White on Red
White on Orange
White on Black
Red on Yellow
Green on Red
Red on Green
Blue on Red (least contrast)
Both extremes of the list are worthy of note. Yellow on black is far more visible than the conventional white on black. Red on green and blue on red are combinations of dark colours with low relative contrast which will create a visibility problem. The red/green combination is also the one which people with colour deficiencies have the greatest trouble differentiating.
Primary colours are those used to create other colours. Some special colours cannot be produced by mixing the primaries. Special coloured inks are printed for this purpose, for example, the brown colour in Figure 13.9. There are three conventions in common cartographic usage for defining primary colours:
| i) | artistic primaries; |
| ii) | additive primaries; |
| iii) | subtractive primaries. |
The additive and subtractive primaries are sometimes referred to collectively as optical primaries.
Artistic primaries are commonly defined as blue, yellow and red. From these colours, most other colours can be created with inks or paints. Thus mixing yellow and red produces orange, red and blue results in violet and blue and yellow gives green. Intermediate colours are obtained by varying the strengths of the hues. Artistic primaries are based on the subtractive principle (refer to Section 10.6.3). Each tint or pigment absorbs some part of white light and the perceived colour is what is left over. Artistic primaries are used in mechanical colour separation, the conventional cartographic method of obtaining various colours. Their use has the disadvantage of requiring many separation overlays but the resulting colours can be easily controlled.
The blue, green and red bands may be referred to as one-third colours, each comprising approximately 1/3 of the visible spectrum. When white light passes through a prism, the blue, green and red colours cannot be further subdivided, hence the term primary colour. When correct proportions of blue, green and red light are projected together, the other known colours can be created. Also, in the correct proportions, a mixture of all three produces white light. Thus the one-third colours blue, green and red are known as the additive primaries. A complete colour system is possible from the three additive basic or primary colours.
Projecting two of the one-third primaries together creates new colours which are of distinct interest. Red added to blue projected light creates magenta. Blue added to green projected light creates cyan and red added to green projected light produces yellow. Cyan, yellow and magenta are known as two-thirds colours, each containing wavelengths equivalent to 2/3 of the visible spectrum. Thus the full sequence of optical colours is red-yellow-green-cyan-blue-magenta and back to red in a circular fashion (Figure 10.3).
Additive primary colour mixing is the basis for optical colour separation used in the reproduction of full-colour originals which are used increasingly in cartography. It is also the basis for preparing transparencies for use as projected overlays onto a screen and as the basis for colour television.
Cyan, yellow and magenta, the two-thirds colours, are the subtractive primary colours. Superimposing filters of these colours over a white light source selectively eliminates parts of the spectrum, in contrast to the additive process described above. A combination of cyan and yellow filters used to filter white light produces a green hue. Magenta and cyan filters result in a blue hue, and magenta and yellow filters superimposed in a similar manner produce a red hue. Thus by subtracting light (filtering), the additive primary colours are created. Using cyan, magenta and yellow filters together will remove all available light resulting in black.
Figure 10.3 Colour wheel showing duos and triads of complementary colours with additive (caps) and subtractive (lower case) primaries.
The subtractive system is the basis for printing with the process colours cyan, yellow and magenta used together with black to sharpen the image. It is also the principle used in mechanical separation, the conventional cartographic method of creating colours on maps and graphs. As with artistic primaries, the subtractive primaries have the disadvantage of requiring many separation overlays but the resulting colours can be easily controlled.
Colour harmony, a visually pleasing arrangement of colours, can be achieved by the use of complementary, analogous or monochromatic colours:
Colours complement one another when they contain approximately equal visual amounts of each of the three pigment primary colours. A colour wheel is a considerable aid to their identification and will be referred to throughout this section (see insert). The sequence of colours on a colour wheel is itself considered to be harmonious and may form the basis of useful colour scales. Two colours (duos) are harmonious and complementary if they lie opposite each other on the colour wheel. Thus we have the following duos of complementary colours with the artistic primaries (Figure 10.4):
Yellow - Violet
Yellow - orange - Blue-violet
Orange - Blue
Red - orange - Blue-green
Red - Green
Red-violet - Yellow-green
The same principle holds for groups of three colours on the wheel (triads) (Figure 10.4). Thus we have the following triads of complementary colours:
Yellow - Red - Blue
Yellow-orange - Red-violet - Blue-green
Orange - Violet - Green
Red-orange - Blue-violet - Yellow-green
The principle of harmonious and complementary duos and triads of colours in relation to the artistic primaries can also be applied to the additive and subtractive primaries (Figure 10.3). These colour duos and triads are more harmonious if the colours are lightened by white, darkened by black or similarly toned down to a pastel shade by adding equal amounts of grey. Pastels or subdued colours are considered to be visually more pleasing than pure colours.
Brown, a fundamental cartographic colour, is not listed above but is often used in many mapping products. Brown essentially consists of yellow and red plus a small amount of blue. Harmonious colours for browns are found in those colours in which the dominant pigment is weakest in the brown. Thus we have the following complementary colours for brown:
Yellowish brown - Blue-violet
Reddish brown - Blue-green
This system of obtaining harmony between colours is extremely valuable to the thematic cartographer who wishes to produce a scale of colours for related features, e.g., the depiction of varying levels or densities of a species.
Analogous colours use one quadrant section of the colour wheel, moving from one extreme steadily through each neighbouring hue to the other extreme (Figure 10.4). Thus a scale of harmonious colours might include green, yellow-green, yellow and yellow-orange. These scales are often adjacent to the subtractive primary colours magenta, yellow or cyan. When using this system, extending beyond 1/3 of the wheel will result in unharmonious colour combinations.
This system is useful as an economic solution to costly colour printing. Using only two basic colours in varying saturations and overprinting them can create a useful scale of related colours.
This is an economical colour system based on a sequence of several continuously graded colours of a single hue. These gradational sequences are achieved by the addition of white, grey or black, resulting in desaturated, pastel or shaded series of colours.
The system is suited to displaying continuously variable data such as water depth in relation to bottom topography.
Figure 10.4 Colour wheel showing duos and triads of complementary colours with the artistic primaries.
