I usually use a medium brown when I do this technique on a light fabric. If you use something that is darker than the fabric, it makes the stitching line seem to sink down further into the fabric, popping out the trapunto even further. If you use a lighter thread, it will still pop out the exact same amount, but the optical illusion isn't there. I would think that the cranberry on the blue would work...its a little darker shade, isn't it??? If you don't use the cranberry...use a darker shade of blue...or even a darker brown type tone. Maybe you should do a "test run" on a scrap first to see which you like the best.