"One of the first things I discovered was that the amount of color you see is a result not so much of what you look at--but of how you look at it. To help you understandthe idea, we should first examin how the eye operates.
"The eye sees through color cones. It has three sets of cones: one for red, one for blue, and one for yellow. From these colors, it creates everything you see in the world. For the eye to be excited and stimulated--and for pictures to have snap, the viewer should see a bias toward the red, blue, or yellow in your color. The eye should be able to dissect the colors. That is one reason black and the earth colors have little effect on the eye. Black, for example, is opaque. It's just a black and the eye can't decipher it, can't break it into recognizeable color parts."
Gruppe here is probably projecting his own explanation for the observations he has made in his painting. M perspective, based on my research into how the eye works, is that the eye is indeed much more stimulated when there is a bias in a color toward red, yellow, or blue. But this has more to do with how the brain interprets stimuli from the cones. Cones are connected in groups to the nerve cells behind the retina in such a way that the eye/brain is always making comparisons rather than measuring absolute levels of color. The brain also is more interested in variations and change in the environment. so a flat field of color is less interesting than a varied one, and a color with no bias toward red, blue, or yellow is equally stimulating to all the cones, therefore less interesting to the eye/brain. Because research is continually turning up new information, even this perspective may be outdated. But the useful part is: stimulate the eye/brain. Creat variation. Create colors that have bias.
Black may still be useful for creating contrast with another color. Black is also useful when the entertainment of the design is not in color, but in shape. Earth colors can be useful as long as they are varied and not used straight out of the tube.