Think about visual comfort as well, some people find certain colour combinations hard to look at. I'm going to be doing a yellow wholecloth eventually, where the backing is a pretty flannel in an owl pattern in cream/golden yellow/orange/pink/purple. I was originally thinking red, but when I tried it against the fabric, I didn't like how it looked and I found it rather unpleasant to stare at. I then tried a couple of deep pink threads, and despite being an avowed pink-hater, found that I preferred the contrast and that they generally looked nicer. So just keep playing around with samples and thinking about what you like in terms of how it really looks, as opposed to what you think should work.