There are three kinds of color sensitive cones in the human eye. In an experiment reported in 2005, scientists were able to examine the retinas of subjects and determine how many of each type of color receptor were present in a small area. They had each subject adjust colors on a monitor, using electronic controls, to display the exact color he or she considered to be yellow. All subjects selected a nearly identical combination, according to the controls, yet the number of each type of receptor found in their respective retinas varied widely.
According to research published in 1965, nearly two-thirds of the cones are sensitive to red, almost one-third to green, and only about two percent to blue. The 2005 experiment showed that this is not so for every individual, with each person examined in this more recent experiment having a unique ratio of the three types of cones.
Another study had people from many different cultures, speaking many languages, classify 320 color samples. Among the subjects selected for this study were people from pre-industrial cultures; researchers hoped to include individuals whose color classification systems were not influenced by definitions of colors used in international business. Surprisingly, people from the various language groups grouped the color samples in similar ways. Scientists concluded that color perception occurs predominantly in the brain, not in the eyes.
Researchers in the science of color perception also noted that there are different ways to combine wavelengths of light to arrive at the same perceived result. Each precise hue that humans classify as the same can be produced with more than one “recipe” of wavelengths. That is considered further evidence that color classification is done by the brain in response to input from the eyes.