There are 45 breeds of rabbits recognized in the ARBA and a few in the process of being recognized. Each breed has a certain number of varieties (colours) that it can be shown in. Some only can be shown in one, others can be shown in over 20. It all depends on the breed standard. Some breeds are distinct because of their colour pattern, so there would never be another colour showable in that breed. For example, Californians are distinct in their white body with black ears, feet, legs, nose, and tail. The standard is written around that colour, and it won't change. The only possible addition would be maybe to allow the colour points (ears, feet, etc.) to be blue-grey, chocolate-brown, or lilac (a light grey) as well as black. In breeds where there is no specific colour attributed to the breed, any colour can become showable if it goes through the process of getting accepted.
Mini rex are showable in black, blue, broken (any colour with white spots, also includes tricolour which is like calico), castor, chinchilla, chocolate, himalayan (black and blue point himalayan are shown together), lilac, lynx, opal, red, seal, tortoise, and white. Otter, blue eyed white, sable, and sable point are all currently being worked on for acceptance.