All colors are either red based or black based. Bay and black horses are black based. Chestnut horses are red based. Grey horses can be either, because the grey masks another color, such as bay or chestnut. These horses are almost never born grey, the horse is born one of the other colors and then they "grey out" as they get older.
Grey horses are often confused with white horses, but see the black muzzle and the black around the eyes? A true albino has pink skin.
Grey horse (either base):
Here's a chestnut horse (red based):
Here's a true, or jet black horse (black based):
And this is a bay (black based):
One of the reasons that a bay turns out bay instead of black is because of a gene called Agouti. If Agouti is present in a horse's coat, it restricts the black to the legs and muzzle, so the horse is bay. If there is no Agouti, then there is nothing to restrict the black so it covers the entire horse.
Horses are either homozygous or heterozygous for any gene that they have. Homozygous means that a horse has a double copy of the gene, and will always pass on one copy of the gene to its foals. Heterozygous means that the horse has only one copy of the gene. For example, a horse that is DNA tested and shown to be e/e is homozygous for red, and will never produce a black based foal. But a horse that is DNA tested and shown to be E/E is homozygous for black and will never produce a red based foal. A horse that is tested to be E/e can produce a red based foal or a black based foal, because it has one copy of each gene. However, since black base is dominant and red base is recessive, a horse that is E/e will appear to be black based.
These are just the basic horse colors. You can add dilutions to them to get other colors, but we'll talk about that next time.
**NOTE: I got all of these pictures and all of the pictures I will be using in the rest of the horse color series off of Google.**
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