What I did when I had that problem was take pictures of several of my blocks straight on, so that they look as square as possible in the picture, and then I tested different colors by "pasting" rows of them in Windows Paint (which is usually included on every Microsoft computer). Then the background color can be changed, and you can see for yourself what the effect might be. Here are some examples of one. The blocks don't look as square as they are, but it doesn't really matter for this purpose. I tried several others, but wound up choosing a light green. Haven't sewn it yet.