You are making it more difficult..... Don't you hate when that happens? Since you won't see it, I don't think it matters. But, if it were me, I would probably use the blue, because if it does show, it will probably be against the blue fabric and if you see the red on the blue, it will be bright. I have started sewing with a 1.8 stitch length and my thread barely shows at any time in any seam.