It is very common for tails (and manes for that matter) to be different colors. My mare has 3 different colors in her tail-dark red, medium red, and white. Her mane is 4 different shades of red mixed with white, and then her forelock is a medium red that isn't like any other red on her body. The black hairs you see are probably part of his winter coat that he hasn't lost (depending on where you live). My friend's bay changes from a dark mahogany bay to a light cherry color when his winter coat comes in. Horse genetics are weird, as are coat colors. I would personally call your horse a chestnut from what you describe. If you post pictures we can tell you 100%