Primary Colors are basically the colors that are considered as the basic colors of universe. These colors are usually mixed to make different other colors. The humans have only three kinds of cone cells which act as color receptors. That is why humans are known as Trichromats. Humans, thus respond through a three dimensional sensation that is why there are only three primary colors.
The three primary colors are:
These primary colors are mixed to make other colors such as Red and Blue are combined to make shades of yellow, brown etc. Blue and green are mixed to make cyan etc.