As you say so neatly, the intersections between class and race obscure much. But my instinct with this is that it's mainly a class thing – only because over here, when I think of stereotyped over-expressiveness, I am most likely to think of a good-ol' chav. But here colour does not have the class associations it does in the States (which is why we have to rely on accents and other shibboleths for our casual discrimination).