Hi, welcome from North Carolina. You'll get expert advise on any questions you might have here. I agree with the medium or even light grays for the piecing, but are you asking about the quilting? Are you planning to stitch in the ditch? That's usually the way most beginners do their early quilts. ... quilting right on the seam line. It isn't really meant to be visible. Then the med.gray would blend in fine there, also. Just my opinion. The important thing is that you enjoy your project and have fun!