Tu aimes le violet ? Oui, ________ est très joli! (Il vs c'est)

Here is a rule that's better than 80% correct: When the verb is être, use c'est and not il/elle est.

The explanation here is that the question is about the general color purple and therefore not a specific reference to anything in particular. But if you go with the rule above, you'll get it right almost every time.