There are only two pigments involved in coat color; red and black. Every animal starts with the potential to produce both. There is one gene that either allows the horse to produce black pigment, or pretty much shuts it down, so that the only color you see is based on the red pigment only. This is what people mean when they say a color is red or black based. There are a lot of other genes that determine how much pigment is produced, and where it appears, but the starting point is red/black. Depending on what other modifying genes are present, it might be impossible for two particular black-based horses to produce black-as-ink offspring, but that isn't what we're talking about here. They can produce some black pigment (Eumelanin), therefore, we refer to the color as black-based.
Bay, for example, is a black-based color. At the red/black location, the horse begins with a gene that says that yes, this horse can produce both red and black pigment. At another place in his genetic code, he has the agouti gene, which says that the black pigment will not appear on his body, only on the legs, mane, tail, and tips of his ears. Add the results of these two genes together, and what you have is a bay.
Agouti is only one of many genes that modify how much pigment a horse may produce, and where that pigment may appear. Your mare appears to be expressing a whole slew of modifiers. As Freeland said, a greying-out silver buckskin has:
Black - which gives the horse the ability to produce both red and black pigment
Agouti - which allows the black pigment to appear only at the points
Creme - a "dilution gene" that dramatically reduces the total amount of both pigments that a horse can produce
Silver - a gene that reduces the amount of black pigment produced, most noticeably in the mane and tail
Grey - A gene that gradually shuts down the pigment producing cells in the horse's skin, so that the hairs become colorless. Grey does not erase the rest of the genetic instructions, it just interferes with the ability to express it.
(Dun) - another gene that dilutes color, but has a very specific pattern of areas that are left darker (Tommy is a classic example)
When you add in the fact that foal colors can vary pretty wildly from the color of the adult coat, I ain't gonna even try to guess what color your little guy is!
