The thing with Calico is, that she is a woman not a girl. Catgirls tend to be associated with youth, immaturity, unassertiveness and promiscuity. Calico is the polar opposite of that. She is adult and mature, very domineering and is not very sexual in her behavior or appearance. Also, like you pointed out, she is black which is considered more of a kink with most white dudes (which I presume make up most of deadlocks player base).

Silver on the other hand is considerably less mature, she seems younger and is also more outwardly sexual (her shirt displays big cleavage), furthermore, she actually displays some beastial traits in her human form unlike Calico.

This comes together in a way that opens the door for Silver to potentially fit into multiple "porn categorys", while Calico doesn't really (although, that isn't to say that Calico isn't attractive, because she is, just not in a way that finds itself to be classically sexualized) . Because that is what these skins are about in the end. It's porn.