Any color in the visible spectrum can be made up of red, green and blue. Normal people can identify three primary colors, while some person cannot distinguish between colors, they are called color blind. Red-green blindness can't tell red from green. And for total color blindness, the colorful world in its eyes is grey, as much as the concept of black and white TV, only the difference between light and dark. But sometimes, color blind to color intensity for resolving power is very strong, and they are more careful than normal color vision, they can see the nuances of things.