No, It is not a sexual thing. Sometimes chameleons go through a color morph for whatever reason. As far as not eating as much, he could be bored with his food as they are known to go on hunger strikes. It is kind of strange though that his appetite has diminished since his color change. Is there anything in the room that has changed? Can he see something now that he could not view before? Did you bring another chameleon or another animal into the room that he can see?? Whatever the case, he sure is beautiful!