Another idea might be to make color grouping quilts or squares. Put all your reds, yellows, greens, etc. together and add a neutral for contrast.
When grouped by color, all sorts of things will go together that you wouldn't think do otherwise. You can include every shade and tone, and this just adds interest to your quilt. Prints with other colors work fine too, just so the dominant color is the one you are working with.
You can make a whole quilt in one color group, or you can go for a rainbow effect using blocks of different color groups. If going the rainbow route, you may not wish to include the neutral.