C'est vs elle est

Bonjour Ann !

Actually, here it's an ambiguous case indeed !
"Tu aimes la couleur violette ?" could be a general question, like "Tu aimes le violet ?" = Do you like purple?, in which case you would use "c'est joli";
but it could also be understood as "Do you like the colour purple [as opposed to other offered colours]?", in which case you could use the more specific "Elle est jolie".

In this question, the second option wasn't possible, as the adjective was given in the masculine form (joli), hence it couldn't be "elle".

But to remove the ambiguity, I decided to change the sentence slightly, to:

Tu aimes le violet ? Oui, ___ est très joli!
Do you like purple? Yes, it's very pretty!

I hope that's helpful!
Merci beaucoup et à bientôt !