I use invisible thread on t-shirt quits usually - you can get polyester monofilament from Superior Threads and it's AWESOME stuff, you can dry it and iron it and it doesn't melt.
If you're heart is set on white on top I'd definitely go with gray for the bobbin. After washing and wear those peeps can relax into loops and become more obvious.
Good luck!
Edited to add - I just saw your repost about going with the gray, so happy it worked out well for you!!