So I am making a UK style numberplate, and I was wondering if there was a way of having white front number plates, and yellow rear number plates, as they both come out as white.
That's relatively difficult work compared to skinning the plate. The license plate skinning system is also likely to change soon-ish; it's definitely in an unfinished form now. This most likely means that if you're asking "how" right now you will not be able to get a good handle on everything required to do the mod before the license plate skinning system is revamped, potentially trashing the work you do. I recommend posting a suggestion concerning different front/rear plates in the "Ideas and Suggestions" subforum - now is an ideal time to do it since the system is probably currently or soon-to-be under construction.