The polecat has a different winter and summer coat. The only trait the two coats have in common is the white undercoat. The winter coat is very white with a long brownish black overcoat. The summer coat is even more brownish black, but still with the white undercoat.
The ferret is a tame and more white version of the polecat. The polecat (and the ferret) can be made pets, but they need a lot of space and care.