Our Nokota / Welsh mare we had used to do that.
In the winter, she would look black (especially from a distance), because her grey color turned so dark.
When she shed off in the spring, the dark hairs turned bleach white, and then shed off.
In the summer (her "normal" color maybe? Haha) she was a nice grey with black points, and even had just a hint of red hairs that would grow into her coat.
In the fall, she'd start to turn the dark, dark grey and lose the red.
I don't think I have a photo from every season, but here's what I got:
This was about in August a couple years ago. This was her "normal" color. You don't really see the red much, unless you are up close to her and can see the individual hairs.
Another summer photo: (she is on the far left, of course)
Here's a winter picture (we didn't have any snow that year, really). She is on the right, of course.
And another winter pictures. This was in January or so:
I couldn't find a picture of her from the spring. But her changing to white was always so dramatic! A nice sign of spring though .... you knew the horses were shedding when she turned white!!