If you are talking about background quilting, match the thread color to the background fabric. As far as the type of thread, well, that would depend on a few factors. For example, if you hand quilt, use a glazed thread for hand quilting; it pulls through the quilt layers easier. If you are machine quilting, do yourself a favor and buy the best thread you can afford; it is so worth the extra money. You will have fewer thread breakages, less lint, and better stitches.
I hope this helps and answers your question.