It's a fantasy game, and the genetics here follows only the general rules but do not represent the real patterns and pigments. There are no real animals whose color and pattern can be described with only 2 genes with several alleles each. In general we follow the rules of gene dominance in the multiallelic environment, and some basic gene interaction like supression and codominance, but nothing more as it will make the genetic system excessively and unreasonably complicated. For example, our gryphon genes do not represent the real genetics at all but used to bind the color to the ancestor real avian and feline species which were taken as a base when gryphons were constructed in the genetic libraries. Can you just imagine how the real genetics of such chimera will look like? The whole genetic chart willl be too small to describe only their colors :) Unicorns are not more real than gryphons, and their genetics too. Just don't take it so serious :)
http://en.wikipedia.org/wiki/Piebald
Piebald in general does not refer to the main coat color, be it black or some other one. It describes only the presence of the unpigmented patches. It's not a horse-only term and used for all animals.
