I do a lot of SID, and I would lean towards a light gray, if I were you. You might try it on a sample of your leftovers? I think you might be surprised at how much the thread doesn't show up. I used the clear plastic thread on a quilt a few years ago, and it was okay, but it was scratchy feeling. I didn't put it in the bobbin though....I think it was someone here on the QB who suggested that. It wasn't hard to sew with, I just didn't like it enough to use it again.
Dina