A rabbit is not considered purebred unless all the rabbits in a 3 generation (parents, grandparents and great grand parents) are all the same breed. So if you took one of these babies and bred them to a purebred of say the mini rex, you would have to wait until the netherland dwarf rabbit gets off the 3 gen pedigree before you can consider them purebred.
Since the rabbit registration system is different than dogs, it is possible to register rabbits who have had cross breeding in the background. With dogs, puppies are registered quite young, usually around the time they go to new homes. They can only be registered if the parents are registered with that club. So if a puppy was a mix, even from purebred registered parents, the puppy can't be registered. Since that dog is not registered, none of it's offspring can be registered either. With rabbits, registration is based on the individual. They must have a full 3 generation pedigree, it does have some requirements as far as needing tattoo numbers, weights and other info for each rabbit in the pedigree. The rabbit must then be evaluated to determine if it meets the breed standard. If everything is in order, the rabbit is registered. A rabbit does not have to have any ancestors registred, but if there are different seals that can appear on the pedigree.
Most of the time cross breeding is done for a purpose. It can be to help with too much inbreeding in a breed, to get new blood into the breed. Some can be done to improve the body type of some rabbits. Creating new breeds requires careful mixing. New colours may also require some mixing. Usually the breeds used do work with each other, you wouldn't cross 2 totally different breeds just because one has a colour you want when there are more suitable breeds to use.
You'd have to get more rabbits in order to outbreed and create what you were striving for. 
