There are two base coat colors of horses: red and black, and these have overlying restrictions and dilutions, giving us a wide range of the colors we see in horses. Further, restrictions limit where the black is either fully seen, partially seen, or not seen. Dilutions are then added to the base colors to give us variety. Overlying all of this is the gray gene that dominates all colors and patterns if the gray gene is present.
Patterns of white spots, patches and hairs make each horse an individual and even a breed, as some share these patterns genetically. Genes determine five groups of white markings, and random markings become unique in identifying all horses not associated with specific genes.
The order goes like this – first is the base color of red or black. Next is a gene that allows black coloring on red horses. Next is the gray gene that can turn any colored horse gray with age.
The next layer for some horses is the dilution genes of cream and dun. These will dilute to become buckskin, palomino, gruella and others.
Over the base color, with or without these dilutions, some will add patches of white, mixes of white hairs or white areas with spots.
Finally, over any of the above bases, expressions, dilutions and markings, individual markings like a star on the face or a small white sock on one leg can randomly occur. The idea here is an order from the base to the final appearance. Just follow the order.