What color does buff and black make?

First of all, you have to define what buff is and exactly what red you are using.

Buff can be a very pale yellowish pink. It can also be slightly more orange or brownish yellow. Because all of these contain yellow, I would anticipate that any red you use would become slightly more orange.

The best thing you can do is test it on your pallet, varying the amounts of each color to see what varieties you get.