C'est or Il/Elle Est

Bonjour Meghna, "La couleur violette" is a noun (couleur) followed by an adjective (violette) so this counts as a noun and is why c'est is the correct response. Also, the fact that the last word is joli (masculine) rather than jolie (feminine) is another hint: c'est is always followed by a masculine noun, whereas if the subject were elle, it would have to be jolie.