Mine frequently change over the seasons ? bay is rather dark over winter and goes bright bay in the summer. Shetland (palomino) goes very pale over winter and darker in the summer. Grey is white white over winter then flea bitten in the summer, and the spotty one is just constantly changing!
My min Shetland had a black winter and summer coat but went through grey while changing in spring and autumn. I think his base shorter hair was always white and the longer hair was black so when that fell out you could see the shorter base layer.
I had people stop to ask if I had a new pony when they weren't used to him changing and they didn't believe me when i said it was the same one