I have to agree with the light or medium gray thread for the quilting unless you could find just the right variegated to go with all these colors. And I also agree with laying the possible choice of thread on the top to see how it looks. This usually helps me make a decision. Quilting wise I think I would echo quilt lines several times around in the middle of that four patch area between the melon slices to emphasize it and them maybe even quilt the melon slices like leaf veins. That may be too much for you or the quilt though. Good luck!! Oh and lovely quilt and nice work!!!