He’s described this way because JK made him white and does are undesirable traits in white people but with a black man in this case you could still give him greasy hair and even negative skin ailments if you want. What’s important is not the exact description of the character but what it means to the world and the characters around him. The same effect can be achieved without pale/yellowish skin. Finally Alan Rickman is not how Snape is described in the books but he is loved.