Originally Posted by Captain Evil
I saw a beautiful half Canadian filly for sale who was black with a white tail. How can you tell if it will grow out black or is it possible that it will remain white? Is a genetics test the only way?
I'd base it on whether or not there's a genetic cause for it- most genes that would cause a white tail show other signs as well. I'd expect silver to affect the mane as well as the tail; "frosting" would only be on buckskin or dun dilutes (and since babies often have dun-like primitive markings that later shed out, you'd have to verify a parent carries dun to be a possibility); a pinto pattern would likely have white elsewhere on the body as well. There are always exceptions to the rule (like a gulastra plume) but that's pretty rare.
Depending on the age of the horse, gray could also cause a lighter tail without other obvious signs. I saw a ~3 y/o when I was horse shopping that looked solid black except for the bottom of his tail- it was a really cool effect, like he had a whitish extension in his tail. On closer inspection you could see some white hairs on his face. Turns out he was going gray, but at a much slower rate than most horses!