It's not speciation. What your brother did was selectively breed rabbits with the alleles that caused their colouration. It's a bit like people with the same hair colour producing offspring with the same colour hair; they can still breed with people with different hair colours.

In fact, there are varying definitions of species. From a palaeontological research point of view, when looking at the characters of an animal you ignore the species altogether; each specimen is treated individually and relationships established from character matrices. When this is finalised then you can look at species.