Why Are Colours Arranged in a Circle? A Diagram That Is Partly Wrong and Still Useful

Wavelengths of light form a line running from long to short, with red at one end and violet at the other, and there is no physical reason for those ends to meet. Perception joins them anyway, because the sensation of purple is produced by stimulating the long and short receptors together and has no single wavelength of its own, which closes the sequence into a loop. Arranging hues around that loop puts perceptually similar colours next to each other and puts pairs that cancel one another opposite, which is what makes the diagram useful rather than merely decorative. The arrangement is therefore a map of perception rather than of physics, and treating it as a statement about light rather than about seeing is the source of most of the confusion around it.

Several arrangements exist and each is correct for its own purpose:

• •The traditional artists' wheel with red, yellow and blue as primaries, which is historical and does not mix accurately
• •The subtractive wheel used in printing, with cyan, magenta and yellow, which describes mixing pigments and inks correctly
• •The additive arrangement for light, with red, green and blue, used in every screen
• •Perceptual systems arranged so that equal distances look like equal differences, which the simple wheels do not achieve
• •Opponent arrangements putting red against green and blue against yellow, reflecting how the visual system encodes colour
• •Each gives different complements, which is why sources disagree about what is opposite what

Colours opposite each other on a well-constructed wheel have real perceptual effects, which is why the concept survives the muddle over which wheel to use. Placed beside one another they intensify each other, making both look more saturated than they do alone, which is a contrast effect of the visual system rather than a property of the pigments. Mixed together they cancel towards a neutral grey or brown, which is why a touch of the opposite colour is the standard way of dulling a colour without making it muddy with black. Staring at one and then looking away produces an afterimage in the other, which is direct evidence of the opponent encoding in the visual system. And a small area of the opposite colour in a composition draws the eye with a force out of proportion to its size, which painters have exploited deliberately since long before anyone drew a wheel.

Hue is only one of three things a colour has, and a flat ring shows only that one. Saturation, meaning how pure or how greyed a colour is, is normally shown as distance from the centre, which turns the ring into a disc. Lightness needs a third dimension entirely, which is why serious colour systems are solids rather than circles, shaped like a double cone or an irregular lump depending on how faithfully they follow perception. The irregularity matters, because the most saturated yellow available is much lighter than the most saturated blue, so a symmetrical solid misrepresents what pigments and screens can actually produce. Most practical decisions in painting and design involve lightness and saturation at least as much as hue, which is the main reason a flat wheel is a starting point rather than a working tool.

The red, yellow and blue arrangement taught in schools is historically important and demonstrably wrong as an account of mixing. Mixing those three pigments cannot produce the range that cyan, magenta and yellow can, which anyone can verify by attempting to mix a clean bright cyan or magenta from them. The theoretical basis was worked out in the nineteenth century and printing adopted the correct set long ago, which is why a colour printer has those cartridges. The traditional wheel persists in teaching partly by inertia, partly because the pigments were what was historically available and stable, and partly because it works well enough for the decisions painters actually make, which concern relationships between colours rather than precise mixing. Knowing it is an approximation rather than a law is the useful thing.

Wavelengths form a line and perception closes it into a loop, because purple has no wavelength and is produced by stimulating both ends together. Different wheels exist for pigments, for light and for perceptual uniformity, which is why sources disagree about complements. Opposite colours intensify each other side by side, cancel when mixed, and produce afterimages in one another.