That's a great quilt for your first (or any). I like your color choices. I've quilted a lot of "I Spy" quilts, which of course, have multi colors. For top thread, I've used Gutermann 111 (available at JoAnn). It's an invisible thread that's 100% polyester. There are some people who are averse to clear thread because of problems with some brands in the past. I figure that I use polyester thread all the time, so why should this be any different?
I would also suggest a wavy line.
bkay