Every artist and photographer knows the importance of the primary colors and the results of their mixtures. Everyone knows that red mixed with yellow produces orange, that yellow mixed with blue produces green, and that red mixed with blue produces violet or purple.... Everyone knows this, but it isn't quite true.......

The human eye has 4 types of receptors, three types of "cone cells" for color-vision, and one type of "rod-cell" for dim-light vision (which mostly responds to lights and darks). These dark/light receptors are the best understood of our visual receptors, but they do not give rise to our color vision. In fact, they are normally saturated in the bright light of day, but allow us to see at night. These rod cells are slightly more sensitive to green and blue light, hence we can often infer color even in dim light (the leaves of a rose bush may appear to be a lighter grey than its red flowers).

During day-light, we see both color and contrast via our three types of cone-cells. Each of these different types of cone cells contain a different type of light-absorbing pigment (they are all chemically related to "rhodopsin"). Each of these pigments absorb different colors of light with differing efficiency -- that is to say, they have different absorption curves.

Each of these pigments can absorb a rather wide range of other colors, and the ranges of absorption are heavily over-lapped. Pigment A absorbs from blue through red light (~460 - ~700 nm), B absorbs blue through orange-red (~450 - ~640 nm), and C absorbs violet through blue (~400 - ~480 nm).

What does this mean? It means that the way in which we see colors is by the differential stimulation of these pigments, followed by something like a mental subtraction process. (Some of this "subraction" is probably "hard-wired" in our optical nerves.) For example, when orange light light hits our retina (about 525 nm), both Pigment A and B are stimulated, but not much C. Thus the different colors that we see are really just sensations.

So, here again is our initial riddle, but re-phrased in scientific terms:

When red light (low-energy) is optically mixed with yellow light (medium-energy), the result is orange light, which has an energy between that of red and yellow. Similarly, when yellow light is mixed with high-energy blue light, the result is green, which has an energy lower than blue, but higher than yellow. That is, the resulting color is always somewhere in-between the energies of the initial colors. Therefore, when one mixes red and blue light, one would expect to get light which is half-way in energy between red and blue... that amount of energy corresponds to yellow light. But that is not what we see, is it? When red and blue are mixed together, we expect to see purple or violet, which corresponds to light which has higher energy than blue or red! This doesn't make sense!?! Or, maybe it does?

Again, the answer lies in the fact that our eyes perceive color as sensations which are interpreted by our optic nerves and brains.

FOR FURTHER READING

1. Rossotti, H.,Colour, Why the World Isn't Grey (Princeton University Press, New Jersey,1983).
2. Kochendoerfer, G.G.; Lin, S.W.; Sakmar, T.P. and Mathies, R.A. "How Color Visual Pigments are Tuned," Trends in Biological Science (TIBS), Vol. 24, August 1999, p. 300-305.