Color triangle

Maxwell was intrigued by James David Forbes's use of color tops.

By rapidly spinning the top, Forbes created the illusion of a single color that was a mixture of the primaries:[5] [The] experiments of Professor J. D. Forbes, which I witnessed in 1849… [established] that blue and yellow do not make green, but a pinkish tint, when neither prevails in the combination…[and the] result of mixing yellow and blue was, I believe, not previously known.Maxwell took this a step further by using a circular scale around the rim with which to measure the ratios of the primaries, choosing vermilion (V), emerald (EG), and ultramarine (U).

Later, he mounted a pair of papers, snow white (SW) and ivory black (Bk), in an inner circle, thereby creating shades of gray.

By adjusting the ratio of primaries, he matched the observed gray of the inner wheel, for example:[7]

Next, he rearranged the equation to express the test color (PC, in this example) in terms of the primaries.