Harlequin is the most difficult color to breed, some of the attached links and charts will help those understand. A dog can carry many different recessives, even though we can see a dogs phenotype we can not visually see their genotype, thankfully we can now color test. As a harlequin breeder the two major genes that we don't want to double up on are Merle or Piebald. Two merle carries can produce Double Merle "MM" puppies, these puppies can be deaf and/or blind and carry many other unseen issues. Piebald is a whitening gene, two carriers can not only produce piebalds (a mismark) they can produce whites/light marked harlequin, because of lack of pigment you can have deafness.
We believe in breeding to standard, crossing colors is not something we agree with if the goal is to produce off color. It is possible if two carry a recessive that they may produce off colors....but an ethical breeder will place these as pets.
An excellent read for studying color genetics
Carrie & Grant Michaelson
firstname.lastname@example.org Dixon, CA 916-417-0003