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Historically, 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.
1965 Jointly with Boris Struminsky and Albert Tavkhelidze and independently of Moo-Young Han, Yoichiro Nambu and Oscar W. Greenberg suggested a triplet quark model and introduced a new quantum degree of freedom (later called as color charge) for quarks.