All colors are formed by mixing the three primary colors: red, yellow and blue. Flower colors, for example, are formed by combining pigment granules of the three primary colors. Owing to the sensitivity of conical cells in the retina, the eye is able to present us the colors as they are really in the nature.
What happens when someone does not distinguish colors? Here’s what. Red rays of light reach the eye, but do not irritate only those parts of the retina that are sensitive to red, but also those which are sensitive to green! Green light works in the same way and irritates the sensitive cells of the retina in red. What is seen in this case? Green or red, but some kind of mix of yellow and grey, because the combination of red and green gets gray. Such a person is only insensitive to red and green, but he or she can see the blue, yellow and purple. Her red and green colors always look like different shades of yellowish, and according to these nuances they can still evaluate when it is red, and when is green color.
Some people may be slightly sensitive to color, even without realizing it. If you are sensitive to red and green, you may learn in early childhood differences between these two colors based on the yellowish color you see. They do not see the color, such as people with normal vision, but estimate the tone and brightness they observe.
Interestingly, the color blind people often fail to conceal their blindness to color even if they are working in jobs that need a normal sensitivity to color, for example, as drivers. Only after determining the cause of many railway accidents, color blind test has become a mandatory prerequisite for performing driver on the railways.