可以这样说吗?what colors are your favorite?

理论上两种都可以.
favorite在前者做形容词,表示“最喜欢的”,在后者做名词,表示“最喜欢的事物”.
也可以这么问：what color is your favorite?
what is your favorite color?
what colors is your favorite?是不对的,colors是复数啊,怎么可以用单数be动词is呢?得用are吖