Oregon and Washington, Tried to look up district info on the AMA site none to be found, but came up with class colors in the general rule book and nothing on the senior class
Contact the promoter of the series you are going to run to see what they require. Generally black backgrounds are for 125/250F, white is used for 250/450F and GP green or yellow for open classes. In my rules concerning background or number colors are not enforced, but some series or sanctioning bodies can be very strict or goofy it.