I change it up all the time on my 928, about every week I pick a random color. Even yellow on white background and that is crazy...
I do miss a full unlock that I had on my WP7.8 Trophy, as there was a app that allowed you to pick ANY color (create your own color or select from 100's of preset ones). There was a BUNCH of custom colors that I used all the time. Wish someone WOULD (and release) a unlock, even if a Interop unlock for 920/928.....
Sigh...