Why can't color blind people see any colors?
Category: Biology Published: July 2, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Assistant Professor of Physics at West Texas A&M University
Most people with color blindness can see almost all colors. They are typically only blind to a few colors. Furthermore, they are not completely blind to those colors, but are only partially blind. The human eye contains three types of color detecting cells: red-detecting cones, green-detecting cones, and blue-detecting cones. Each cone cell can detect many colors in addition to its main color. For instance, the cone cells in the eye that can see red light can also see orange and yellow. The red cone cells just see red the best, and that is why they are called the red cones. Similarly, the green cones can also see orange, yellow and blue. This overlap is what allows humans to see so many different colors. Humans can see the color yellow, for instance, even though there are no yellow receptors, because of this overlap. When you look at a yellow flower, the yellow light stimulates the red and green cells in your eyes, and your brain interprets a little bit of red plus a little bit of green as yellow.
Typically, a color blind person has only one type of light cell absent or non-functioning. The textbook Color Vision, edited by Werner Backhaus, Reinhold Kliegl, and John Simon Werner, states that about 1% of Caucasian males have partial color blindness with one of the three color receptor types absent or non-functioning. This book further states that people who are completely color blind; those with all three types of receptors absent or non-functioning; are "extremely rare". People with protanopia color blindness lack the red detecting cone cells or pigments. As a result, they do not see red or orange colors as well. But they see all the other colors just fine. People with deuteranopia color blindness lack the green detecting cones or pigments, but have their other cones working just fine. People with tritanopia color blindness lack the blue detecting cones. People with these three types of partial color blindness only have problems with a few colors and can see most colors just fine.
You should keep in mind that the response of the different color-detecting cells overlap. When a person with full vision looks at a pure blue wall, it is not just his blue cone cells that are stimulated. His blue cone cells are stimulated a lot and his green cone cells are stimulated a little. His brain therefore experiences pure blue as a large blue cell signal plus a small green cell signal because of the way their responses overlap. Now apply this concept to a color blind person. A person with tritanopia is missing the blue-detecting cells in his eyes. When he looks at a blue wall, his blue cells don't see anything because he is missing the blue cells. But the green-detecting cells are still there, and they see blue a little bit. In this way, a typical color blind person is not really completely blind to any color. But his brain experiences a small green signal as dark green. A person with blue color blindness therefore experiences pure blue as dark green. His color perception is not complete and not quite right, but he does experience color, even in the color range where he has greatest difficulty. Whereas a person with full vision sees the colors green, blue, and violet, a person with blue color blindness sees these colors as green, dark green, and darker green. He sees color. He just sees the colors slightly wrong and has a harder time telling certain colors apart.
Another point to keep in mind is that most colors that we see in everyday life are a mixture of fundamental colors. For instance, in terms of fundamental colors, pink is a mixture of a lot of red plus a little of orange, yellow, green, blue, and violet. When a person with blue color blindness looks at pink, only the blue part of the color mixture is faulty, which is a small part of the color pink. As a result, pink looks nearly normal to a person with blue color blindness. The same goes for other colors that are a mixture of fundamental colors.
Also, there is more to an object than its color. An object also has brightness, shading, texture, and shape. If a chair that is painted pure red is placed in front of a person with red color blindness, she would have no problem seeing the chair. She would still see the shape, shading, texture, and brightness of the chair. She would still see some color in the chair too. The chair would just look dark yellow instead of red. In this way, people with color blindness are not really "blind". They can see all objects just fine. Better phrases would be "incomplete color vision" or "less sensitive color vision".
Finally, people with color blindness learn to use mental tricks to compensate for the deficiency. For instance, oranges are always orange and stop signs are always red. A person with red color blindness has a harder time telling apart red and orange, but he can sure tell the difference between an orange and a stop sign. Even though the color difference between these two objects is very subtle for him, he feels that the color is very different because he associates the object identity with its color, and stop signs are very different from oranges. If you place a pure red posterboard next to a pure orange posterboard, a person with red color blindness may have a hard time identifying the red object. But place a stop sign next to an orange and ask him to identify the red object, and he will have no problem pointing to the stop sign as red. He may even think he is actually seeing red in this case, when he is just mostly seeing an object he has associated with the word red. Similarly, a color blind person typically has no problem driving and obeying traffic lights. You may think that he can't tell red lights from green lights and won't know when to stop or go. In reality, he may not see these lights in their full color, but he can see enough color to tell them apart. Additionally, he comes to associate the top light with red and the bottom light with green without even realizing it. He may tell you he is "remembering the color", when in reality he is remembering the location of the traffic light that means "go" and "stop". In this way, color blind people sometimes think they are fully seeing colors even though they are not because they are unconsciously relying on mental tricks and associations to compensate.
This post is meant in no way to trivialize or underestimate the burden that color blind people suffer in having incomplete color vision. Rather, this post is meant to simply uncover the science behind color blindness. Because of their sensitivity to most colors, the overlap of their cone cell response, and the other visual cues connected to an object, most people with color blindness have a nearly complete color experience. (Personal note for those curious: I am not color blind but personally know people who are).