David Brewster was an English physicist known within color research as being responsible for a theory of light which held that each portion of the spectrum was actually composed of three individual types of light which had the primary colors of red, yellow and blue. Brewster spent time researching color vision in order to study the best methods for manufacturing the colored sunglasses that were very popular at the time. Thus Brewster made his observations using filters, a subtractive process for altering the spectrum of light.
Brewster's theory of the physics of light was refutable on two counts: his reliance on colored filters which led him to pick subtractive primary colors, and Maxwell's soon to be discovered relationships between energy, frequency and index of refraction. Although Brewster's work is not considered in high regard by many, he was the first color scientist to forcefully argue the point which later became known as metameric substitution: three properly chosen colors of light when mixed in careful proportions are all that are necessary to reproduce all color sensation.
Brewster's observations were accurate and for that von Helmholtz later gave him credit[von Helmholtz 1852]. Although Brewster thought he was plotting curves describing the physical composition of light, since his theory was based on observations made by the eye he ended up plotting the first approximate trichromat color sensitivity curves [Brewster 1831]. Although the mode of the middle frequency curve was centered on the green part of the spectrum, Brewster continued to maintain that this corresponded to yellow light (see Figure 2).
Figure 2. Brewster's spectral decomposition. Note the sensitivity curves for the three primary colors.