In order to be able to see colors, in the retina, which is the part of the eye where the images are formed, there are some cells called cones, which have three types of photoreceptors that are sensitive to blue, green and red . All the information received by the eyes, after passing through the visual lines and the interpretation made by the brain, allows us to differentiate almost one million colors.
There are people who have the three pigments, but in one tone they are weak and they are often confused at identifying the color of the pigment that fails them. In this group, there are the color blinds. There are even those who only have two or even one. Those who have two are called dichromates. They may have a defect in the green color (deuteranopsy) or in the red (protanopathy). Monochromes only have one. They can see white, black and different shades of gray.
The origin of the defect in color vision is caused by a defect in the X chromosome. Peculiarly, women are bearers and manifest in men. There are 8% of them against 0.4% in women.
Making mistakes in colors can be a problem in certain jobs, combining clothes colors when we get dressed, ...
There are many tests to detect a defect in color vision. The most common is the Ishihara test. A person with a problem in color vision does not see the figure that, on the other hand, it does not suffer it obviously (see the photo at the beginning).