I don't think Deaver ever specifies Rhymes' skin colour in the books, although he makes reference to other characters features (hair colour/style/type, eye colour).
But I think we all do the same thing, whereby an image enters our mind regarding what a character looks like, and we stay with it unless specifically directed not to.
I haven't seen the movie, but Angelina Jolie just is so not the picture I have in my head for Sachs! I have more of a young Sean Young in my mind.