try a medium gray that will blend/disappear more over the colors- when hand quilting you want the stitches to be as even as possible - same distance between each one- about the same distance as the stitch itself- it does take some practice- small even stitches top & bottom- you may want to put together a small practice piece (maybe a 12" square & draw some lines to follow on it- and do some practicing before continuing on your beautiful quilt. a couple squares of muslin with some batting in between is ideal for practicing. once you get more of a (rhythm) to your quilting -with small even stitches i think you will be happier with what you are doing on your quilt- i think the pattern you are doing and a light thread will look very nice- once you get the hang of it.