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In the book, Lorath tends to a mare named Zerae and her baby colt named Arjak. Zerae is described as dapple grey, which is this color, with some variations, and it comes from a base black gene and a graying gene (that gives the "white" color)

Arjak on the other hand is described as white. Now, while not impossible, most horses are not actually white. They are not born white. The horses we know as white, are actually grey, starting off either black or chestnut, and they progressively lose their color until they are very light grey. A lot of times they look dapple for few years until they become "white" (I got to watch on of the young stallions at my ranch go through this). These white horses, that are dark around the muzzle, are not born white, they are born another color and progressively become white.

You can note this because they have dark around their mouth, as grey horses have dark skin.

This is a dapple mare with a little dark baby, but the baby has a high chance of becoming grey or dapple by the time it matures.

However, a horse CAN be fully white, even though it's very, very rare and this might've been done as symbolism in the book (Lorath bonds with the horse and sees himself and his lost innocence in a white foal and is scared the foal will die without his mother, which symbolizes that death of Lorath's own innocence after Malthael's reaping.)

A fully white colt could come if he has a rare combo of dominant white gene, be a very rare white-born foal which carries a very strong white leopard gene that basically covers the whole body in dots so it appears white, or maybe look white, but actually be double dilute coated cremello or prelino. Either way, these horses will have a bit of a rosy pink skin to them, unlike the greys.

(these are white dominant, white-born leopard and a very diluted perlino, in that order. Note the pink around the eyes, nose and stomach)