此句的完整句子是The most common colors which we can see are brown,white and black.
which we can see 是形容詞子句,用來修飾colors, 所以並無二個動詞的問題,因為這是二個句子合成一個句子的結果,主要子句為The most common colors are brown,white and black.形容詞子句是which we can see (在此which因為是受格關代,所以被省略了. ^_^)
The object of the verb is a" that-clause"." That" is often omitted when placed after the subject:-
The most common colors are--------subject +vt
(that) we can see-----that-relative pronoun in restrictive clause after "colors"
brown,white and black--------complement to subject+vt