Grey for piecing.
Both red & black for quilting as I tend toward custom quilting, rather than pantos/all-over designs. If you plan to do a repeating design that runs across both the black & the red fabrics, then it would depend on how confident I was with my quilting design & whether I wanted the quilting design to be the star. Your best bet for getting the quilting to "blend" is probably to pick a very dull red cotton thread in a 50/2 or 60/3 weight. The other option would be a very dull green, since that's red's direct complement & would end up reading as a rich grey, except very close up.
If you want the quilting to be the star, I'd pick a 40/2 white cotton so it shows up equally on the red & black ... although, you could pick either a matching red or a matching black and it would blend on its match & create an interesting pattern on the other color. So many beautiful choices, you really can't go wrong!!!