I might use a rainbow variegated thread and if you have a curve ruler, I would do a curve petal from the seam in the center of the coloured triangle back to the seam and then a curve petal half on and half over the triangle into the white and then a curve petal up to the corner of the white. It would be a 4 petal flower in each square using the center seam in each square as the axis. Clear as mud right? Pretty top!