X-link'ed calico cat question?

So we've been learning about x-linked genetic diseases and every textbook talks about calico cats and their fur pattern. They state that some cells have active orange allele on one X chromosome and some cells have active black allele on the other X chromosome.

But what accounts for the white fur? If all cells have an active X of one or the other X chromosome, how come there are white fur patches also?

Answer probably has nothing to do with X-chromosome inheritance but i was just curious.