Bonjour Daniel ! Actually here the statement refers to the colour purple in general, so in French we will choose the general "c'est" to comment on it. We'll tend to use "elle" if we're referring to a specific object, for example: "Tu aimes la *chemise* violette ? Oui, *elle* est très jolie." (Do you like the purple shirt? Yes, it's pretty.) I hope that's helpful! À bientôt !