I had a buckskin that changed colors too! Only he got darker. I bought him in the winter when he had a shaggy very light colored coat. In the spring as he was shedding out all of his lighter hairs came out first and for a while what was left were these darker long hairs (which i didn't realize he had so many of) making him kind of a smutty color. When he shed out? BEAUTIFUL dark gold buckskin! Faded a bit in the sun as the summer progressed. I sold him and they sent me a picture of him that winter that same beautiful dark gold color. but then I saw him in the summer at a show and he was so dark I would consider him a smutty buckskin! As far as I know they weren't feeding him any kind of supplements, just some grain and plenty of time in the pasture, which is what he got at our place. Weird.