Im not breeding expert either hippy but as long as the two cats are the same predigree breed ie british shortair , you can perfectly easily mix the colours , this is indeed how ya get diversity etc, also in some breed you have recognised outcrosses that you can breed to when the original breed has a very limited gene pool, this is to ensure the gene health and limit inbreeding.
if someone wants to mate their blue british shorthair but wants to have a chance of different colours in the litter then mating to a different coloured british shorthair is fine (as far as i am aware)