Buff has unknown melanin restrictors and can be variable as to exact genotype. For example, some lines of buff carry dominant white, which restricts melanin but has little effect on pheomelanin. You will know if you have dominant white if you get white in the tail or wings when there would normally be black peppering or worse. I don't see much possibility of improving wheaten or blue wheaten coloring by crossing with buff. Or is it buff you would be trying to improve?