4.10 What survives

The rules that fail were made to describe colors by their look and were then asked to predict what pigments do. The eye adds up the light a surface returns in three broad, overlapping bands, one for each kind of cone, and then compares the three signals in two ways. A mixture of paints adds up its pigments’ absorption and scattering one wavelength at a time, and the Kubelka–Munk law turns the sums into reflected light. The first process is behind the three kinds of cone signal, the four hues that look elementary and the circle of hues itself. The second is why a red and a blue make gray, why a dark transparent paint comes alive in a tint, why white adds a faint blue cast to a mixture, and why no three paints are enough. The rules of the wheel mix the two up. What survives of them is modest and useful: that hues form a circle; that lightness, chroma and hue are the three things to name; that a color is dulled by its mixing complement and set off by its visual one, which are often different paints; and that six well-chosen paints reach more than three. Everything beyond that is better read off the reflectance curves, which these notes have tried to show are not hard to read. A painter without an instrument can still see part of it in a paint’s own tint ladder: a strip of the paint mixed with more and more white shows which way its hue moves and where its color is most vivid.