I choose a back color that looks good with the front...I've used both lights and darks. Sometimes I wait until the front is finished to decide. There are times that what works is so different than what I thought would work when I was planning the quilt. Here are a few samples of busy vs. not busy fabric. Busy doesn't necessarily mean lots of colors, just lots of design. The more blank space in the background, the less busy the fabric.
Very busy
[ATTACH=CONFIG]49688[/ATTACH]
Busy
[ATTACH=CONFIG]49689[/ATTACH]
Not so busy
[ATTACH=CONFIG]49690[/ATTACH]