With a 2 color scheme of red and yellow, use a variety of those two colors. The more you can mix it up, the better. Otherwise, you run the risk of having the same fabric 'touching' in another block. (ooooo - - can't have that happen!) I did a YBR several years ago and used too few fabrics. I was kind of disappointed in the result.
We'll be anxious to see your finished quilt and see what you decided!
Jane