I like the idea of using a medium gray.
I like the pattern. What is the pattern name?

Curious, did you use black batting? Beautiful, by the way!

Charcoal.

One of my favorite colors to quilt with is Rail Grey from Signature threads.

Variegated black & white

Since they wanted it subtle, the grey family is best, myself, I would have incorporated red if I knew what I was doing in quilting because that would make the quilt pop. The quilting would have to be good.

Grey for sure!

Silver

That is gorgeous--dark gray or black would look good.

If you do stitch-in-the-ditch quilting, then I would recommend black or white.

If you do quilting that will show, then a dark red would NOT be bright, yet it would add a bit of pizazz.

Do you have a few scraps? If so, try making small sample quilt sandwiches and test a few colors on it to see what looks best to you.