if your mating blacks/choc/reds/fawns together and getting a lots of pied pups there mixed color,all dogs that have white pattern even just a small star might carry the genes for pied, a black over a white producing white the black must be carrying white, as little as a 1/8, five gens of no pied color and your breeding self colors, black- add in dilution gene choc all the way back to cream
all good terriers should be a brown colour sometimes with a pink/red head