Picture of Color Blindness
When we see different colors, we are perceiving differences in the light that is reaching our eyes. The way we see different colors is something like the way we hear different sounds as being low or high. The colors of every rainbow always appear in the same order: red, orange, yellow, green, blue, indigo, and violet. The different colors in each part of the rainbow correspond to a different wavelength of light. Color blindness results from an absence or malfunction of certain color-sensitive cells in the retina. The retina is the nerve layer at the back of the eye that converts light into nerve signals that are sent to the brain. A person with color blindness has trouble seeing red, green, blue, or mixtures of these colors. Most color vision problems are inherited (genetic) and are present at birth. Other color vision problems are caused by aging, disease, injury to the eye (acquired color vision problems), optic nerve problems, or side effects of some medicines. Tests are used to measure the ability to recognize different colors with the most common test being the pseudoisochromatic plate test (also known as the Ishihara color test). The type of color vision problem you have can often be determined by which patterns you can and can't see in the various plates used for the test. In the above example test, the number 74 should be visible within the circle of dots. Inherited color vision problems cannot be treated or corrected. Some acquired color vision problems can be treated, depending on the cause.
Image Source: Wikipedia
Text: WebMD Medical Reference in collaboration with Cleveland Clinic on MedicineNet - Color Blindness