SNAIL used to request that all drivers chose a color and number and stuck to it. With the advent of a full livery editor, that hasn't been enforced or maintained. At the present time, you are free to do whatever you want with your liveries. It would still be nice if you tried to keep from duplicating the number of someone else in the division and better still if you stayed with the same general color scheme so others can identify you quickly on track.
None of this is required any longer but be original.