I've used solid white sashing in some of my more modern quilts, and I've used solid black in one quilt, but in general, I don't think solids mix that well with traditional prints in a quilt (based on the very first top I ever made, in which I mixed a solid with prints and the solid stood out like a sore thumb). I've seen some stunning modern quilts done in all solids, though. Did your DGS specify that he wanted solids?