# Role of quark color with respect to exclusion principle

"Shortly after the existence of quarks was first proposed in 1964, Oscar W. Greenberg introduced the notion of color charge to explain how quarks could coexist inside some hadrons in otherwise identical quantum states without violating the Pauli exclusion principle. The theory of quantum chromodynamics has been under development since the 1970s and constitutes an important component of the Standard Model of particle physics.[citation needed]"

Could anybody elaborate/explain this statement, esp. In light of the "citation needed" note at the end.

In other words, is an intuitive picture possible as to the reasons why quarks come in different colors (or can it only be explained in abstract spin space terms) and can an analogy be given between the Pauli exclusion principle for electrons (arising from opposite spins), and the color charges given to quarks?

In a very basic way, before the discovery of quark color you had a problem with particles like $\Delta^{++}$, since they should have three up quarks. Since the quarks are fermions with $s=1/2$, you could not make a totally anti-symmetric function with them. Therefore people conjectured that a new quantum number called 'color' should exist, and this would be responsible for not violating the exclusion principle.