People with normal vision see a full spectrum of colors. People with color blindness are able to see, but are not able to see all colors correctly.
The genetics of color vision
You may have heard that only men can be color blind. While not entirely true, the majority of color blind people are men. Twelve out of every 100 men are colorblind while only one out of every 200 women are color blind. This is because the gene for seeing color is a recessive gene located on the X chromosome. Women have a pair of two X chromosomes, while men have one X paired with a Y. Because women have two X chromosomes, both X’s must have the defective gene in order for her to be color blind. If a woman has one defective gene and one correct gene, the correct gene wins out and the woman has normal vision, but is a carrier for the defect. Men inherit their X chromosome from their mothers. If a man receives a correct gene, he will have normal vision. If he receives a defective gene, he will be color blind because he does not have a second X to balance out the bad gene.
Rods and Cones
The eye perceives color when light rays strike photoreceptors in the retina of the eye. A photoreceptor is a special, light-detecting cell. The photoreceptors on the outer edges of the retina, called rods, are responsible for seeing black and white images and are more effective at seeing in the dark or at night.
In the center of the retina, photoreceptors called cones are responsible for seeing color. There are three types of cones tuned to a specific color wavelength, red, green, or blue. Color blindness is usually caused by defective or missing cones for one of these three wavelengths of light. Most color blind people have a limited ability to distinguish the colors red and green. In even more rare cases, the problem colors are blue and yellow. The label color blind is somewhat deceiving because people who are color blind can usually see some color. Only in very rare cases does the world appear in black and white.
For some people, color blindness is more a matter of degrees than an inability to see a particular color.