The strong force acts only on certain particles, usually quarks that are bound together in the protons and neutrons of the atomic nucleus, as weel as in less stable, more exotic forms of matter.  Quantum chromodynamics is build on the concept that quarks interact via the strong force because they carry a form of strong charge, which has been given the name of color. There are three types of charges which are known as blue, red, and green; in analogy to the primary colors of light, though there is no connection with the usual sense of color.