I used the homozygous color gene because it is easy to understand. People can see it.
My point is, even if you start with a horse that is homozygous for something, ( in this case color) it is easy to breed that out of the horse. It doesnt take very long at all. If the resulting foal from a homozygous tobiano stallion out of a solid mare is bred to another solid, your chances for color are greatly reduced. Breed that resulting foal to another solid, and your pretty assured of getting another solid. That color has in essence been bred out. You would then if you wanted color, have to breed it to another colored horse. Thus, bringing the influence forward.
So no matter what happens in a pedigree way back there, it is vertually a moot point unless, you bring that influence forward again.