Some horses appear to be a quite different shade in winter to what they are in summer. Why? There are two different reasons:
1. Some darker haired horses bleach a fair bit in the sun. Then the winter coat comes through at their "normal" color and appears much darker. (It's the same phenomenon that gives some brunettes "highlights" towards the end of summer, self included).
2. Horses have an interesting coat cycle. All horses, although it's more pronounced on ponies and horses that are kept in cold climates, have a single-layer coat in summer and a double-layer coat in winter. On some individuals, either the outer guard hairs or the "fluffy" undercoat come through a different color. I used to know a horse who would go from brown to bright red when his winter coat came through, and the Appaloosa mare I ride has a nearly white undercoat which makes her look much lighter.
The second phenomenon is fairly rare, enough to be remarked on, but not so uncommon that people don't know about it.