my guess is bay it is not uncommon for bays to have countershading some even true dorsals I had a stallion like that and he passed that on to all his foals he wasnt a dun though as he didnt have any other traits so he was i am told called a line back bay
anyway all that aside what a VERY cute baby congrats
Sorry, looks too red to me also to be anything but bay. Can't be a dun -one of the parents must be one. Alot of bays are born with shadowing. You do need a creme gene to get a buckskin - possibly the sire is a smoky black pinto - has he ever produced palominos or buckskins out of non creme mares? What color are the sire's sire and dam? If you want to know for a certainty for $25 you can have her tested for the creme.
The beauty of foal coats is that you don't know for certain until they shed that baby fuzz! We have been breeding horses for over 20 years and we still come up with occaisional one that is anyone's guess! I had a red dun filly(red body, head,legs, dorsal that shed out to a grulla with a black head, legs , dorsal and mouse colored body! Then at a year old, she started to gray (dam's a gray so we were expecting that one). I have have sorrel foals (red body, mane and tail) that have shed out to palominos with flaxen manes and tails! Now, with genetic testing it is easier but I can't wait till they come up with a test for dun - especially to determine if they are homozygous dun.
Regardless of her color, she sure is a beauty! Congratulations!