More especifically, it's ok as far as the screen is rendering one single (maybe static) "something", say a full color background or a not moving picture maybe, the thing is it must always be able to tell that it is in one state or the other, not both nor mixed states. That's why we first think of just two full colors, but as it was asked in another thread, i think it's ok to use some animations if you can make the animations be quite distinct to each other and to stop one and switch to the other one cleanly so the above stated holds.
Greetings, MG_.