"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."
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?