My TWH gelding has a mahogany colored body with black legs, tail, and mane. But on his nose he is not black, he is a lighter shade of brown. You can't see the lighter hairs in the summer, they just come in with his winter coat. Someone told me this means he is not a true bay in color? Does that sound right?
Agree with Cat, it's a modifier gene like Dun or Bay. My bay gets a light muzzle in the winter too, as well as his belly by his daddy parts gets almost white. In the summer though, he goes back to his normal mahogany bay color. From what I've read, being a "mahogany" bay, is actually an indicator of the sooty modifier at work. Sooty causes dark hairs to be interspersed through the coat, most heavily along the neck and withers and sometimes the head. When Jack was a baby he was a BRIGHT copper bay, but every year he's gotten darker and now is almost purple looking at times.
The first is a pic of Jack at 2, nice and bright copper, but you can already see him getting darker along the neck and withers. And the second was only a month ago. (Excuse the mud, this was pre grooming, lol) You can kindof see the white fuzzies peeking out by his rear legs.
Your horse sounds like a Seal brown bay color which is a true bay color. There are variations in bays. A bay horse can be any shade of brown (which is a mixture of red and yellow), with points such as tail, mane, muzzle and lower legs being black. The brown can range from a light, almost tan or chestnut to a dark, seal color.