Without seeing both the light and dark, and after looking at this piece, I would think of using the circle and maybe the square would be bigger if neccessary or just a yellow that goes with the light and dark. Yellow stands out the most in quilting and I really like red. IT IS YOUR QUILT YOU DO WHAT MAKES YOU FEEE HAPPY, GOOD, COMFORTABLE. Since you are using peacock maybe a yellowish green square for center.