I'm slowly working my way across the bottom of the quilt, quilting the reef. I'm just over the half-way mark. I use a Rainbows (40wt variegated trilobal polyester) thread from Superior. The mix of leaf green, butter yellow, lilac and hot pink seems to work really well; both blending and highlighting effectively.
It's a bit hard to see in photos; these photos aren't too accurate in colour, but they do show some of the quilting detail.
In some places I closely follow lines on the fabrics, others I quilt something similar to the prints (e.g. the red seaweed above), and other places I make up new things on top of the fabric to help them blend together even better.
The photo above is from the back, and I've edited it so that the thread stands out (the fabric is actually greenish blue, and the thread is a soft green!).