I have a 2 yr old stud colt named Phoenix, his color is a bit of a mystery to me as I rescued him and have no idea what his parents looked like. He started out the yellowish of a buckskin and over the winter changed to the odd golden brown he is now with shading on his legs and along his back, they are no black points and his tail is black/white/black.. here are some pictures of him.
Also his muzzle stayed lighter in the winter indicating he was going to be a bay, but he didn't go anywhere near bay.
Brownskin! My boy's a brown based buckskin out of a perlino mare and black tobiano and looks a lot like your boy. And like yours, he looks like a dark buckskin in the summer and turns really dark in the winter. They're like chameleons, always changing colors. :)
First picture is at 4 months last August. Second is in October when his winter coat started to come in. Third is in November, and the last was just a couple months ago. Chameleons!