WARM AND COOL HUES

Figure 7.17. Two of the earliest representations of cool-warm directions on a colour wheel. Left: "Cold" and "warm" colours from Charles Hayter's Perspective (1813). Right: "Hot" (upper right) and "cold" (lower left) directions from George Field's Chromatography (1835).

The familiar expressions warm and cool refer to psychological associations of colours rather than any physical properties of the light inducing them. They seem not to have been applied to colours until soon after artists first saw their range of hues laid out in a circle. [Charles Hayter's colour circle of 1813 (Figure 7.17) appears to mark their earliest appearance in a published colour system, but the terms can be traced back in artist's correspondence as far as 1727 (Gage, 1999, p. 22)]. The introduction of the terms may simply reflect the fact that, when laid out in this way, it is immediately evident that the range of hues associated with sunlight (yellow to crimson) are confined to one half of the circle, and sit opposite a range of hues centred on blue or blue-green.

The expressions warm and cool are all too often used by artists in a vague sense that fails to separate the concepts of hue and chroma. For example, a teacher may tell a student that "that red area needs to be warmer". This could mean either that the hue is correct but the chroma is too low, or that the chroma is correct but the hue needs to shift towards orange. Either way it probably means that the teacher is not thinking clearly in terms of the three dimensions of colour.

The terms warm and cool can however play a useful role, as long as they are always used in a precise sense referring specifically to relative hue. In this clearly preferable sense, the terms provide a useful means for referring to relative positions and directions around the hue circle.

Since "warm" and "cool" are psychological associations, it is not surprising that there is a great deal of inconsistency in usage. At the extremes some authors, such as Arthur Pope, regard yellow and violet as marginal hues, neither warm nor cool, while others regard yellow as the warmest and violet as the coolest hues respectively. However the commonest positions in the literature seem to be to regard red, orange-red or orange as the warmest hue, and blue or blue-green as the coolest.

If a warm-cool polarity is to be used to describe relative positions around the hue circle, some consistent and rationally justifiable definition would be helpful. Since it is a psychological association that we are talking about here, it may be relevant to note that both the yellow-blue and the red-green opponent pairs have a warm-cool polarity that is unambiguous judged by the criteria of most authors. This could be taken to support a decision to define the warmest and coolest colours as yellow-red (orange) and blue-green respectively, a position close to the average usage. These two hues are of course only directly opposite each other on the psychological colour wheel.

Figure 7.18 Suggested relative warm and cool directions on the three basic colour wheels.

The tendency of warm hues to appear to advance and cool hues to recede has been given a simple and accurate explanation in terms of the physical phenomenon of differential refraction (Luke, 2001). This phenomenon causes the light rays from a red object to focus further from the lens of the eye than the light from an equidistant blue object, at a point where light rays from a slightly closer object would focus. Though not our primary means of depth perception, this effect can give a strong impression that the red object is closer, particularly if it is not contradicted by other sources of depth information (Figure 7.19).

Figure 7-19. Demonstration of advancing and receding colours. Notice that it is not the colour with the greatest brightness (green) that seems to advance most, as has sometimes been asserted, but the one associated with the longest wavelength (red)

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