Theory that describes the action of the strong force .
The strong force acts only on certain particles, principally quark s that are bound together in the protons and neutron s of the atomic nucleus , as well as in less stable, more exotic forms of matter. Quantum chromodynamics has been built on the concept that quark s interact via the strong force because they carry a form of "strong charge," which has been given the name "colour." The three types of charge are called red, green, and blue, in analogy to the primary colours of light, though there is no connection with the usual sense of colour.