There are individuals, known as “colorblind”, who do not see certain shades of blue, green and red. In other words, they have difficulty understanding the wavelengths of light corresponding to these colors.
They do not have photopigments sensitive to these wavelengths. But as a general rule, unless we suffer from color blindness, we can see all the colors of the rainbow in all their shades. Two theories of color vision have been developed: the “trichromatic theory” and the “theory of opposing processes”.
The trichromatic theory is very simple. It states that the retina has three different types of cones (photoreceptors), each of which responds to different wavelengths of light, allowing us to distinguish colors. The first types of cones react to “short wavelengths” (about 440 nanometers), which is blue light.
The second type of cone responds to “medium wavelengths” (about 530 nanometers), i.e. green light. And the third type of cone responds to “long wavelengths” (about 560 nanometers), i.e. red light. When several types of cones are partially activated, we can see variations of these three basic colors, such as turquoise or orange.
All the colors we see are therefore mixtures resulting from the activation of these three types of cones. The theory of opposing processes is that the brain has different types of neurons that react differently to colors. The excitation of one class of cells is higher than normal when stimulated by a certain type of light and lower than normal when stimulated by another type of light. For example, if you look at red, the excitation rate of specific cells of that color increases.
If you look at green, the excitation rate of those same cells decreases, while the excitation rate of the specific cells of the color green increases. There are other categories of cells for yellow and blue. This theory helps explain the “negative post-image effect”, a phenomenon whereby the images you “see” in your mind are not the same color as the real images. Let’s take the example of the French flag.
Let’s replace blue with yellow, white with black, and red with green. If you look at this image for a little while, when you close your eyes, you will see the flag in its true colors. The cells stimulated by the yellow, black, and green will have returned to their normal state and will “see” blue, white, and red instead. Give it a try. Stare at a 2 cm2 yellow square for about 30 seconds, then look at a white sheet of paper. You will see a blue square!