I note those plastic cover for plates "to keep them clean" seem to be more & more popular.

These are not allowed in California (although the chance of getting popped must be quite slim, by the number you see). I remember seeing some water based spray paints in dun colors (such as a nice mud tan) - Gee, officer, its just mud, <wipe>. (Why water colors should be in spray cans is beyond me, unless intended for taggers).