What colour should i breed my Blue/grey rats with?

If you insist on breeding, you may want to research pet rat genetics. It depends what colors you are wanting. If you breed a blue rat to a black you will get all black babies. If you than breed them back, hopefully with a black carrying blue, you will get some different shades of blue. Blue is recessive. Genetics is not an exact science. If you are breeding for fun, or just to see what colors come out, than rethink it. If you are trying to start a breeding program, get some mentoring from an established breeder first......