I have not used this idea however it sounds GREAT....
If you buy a main fabric or if you find a fabric that you just HAVE to use in a quilt take that piece of fabric with you to the local hardware store, or chain store that sells paint. Then go through the paint swatches and find colors that you think will go good. Take them home lay them out then with natural sunlight take a photo of them. I always photograph fabric to make sure they look right together.
Another thing is printed fabrics all have swatch dots on the selvage. These colors are the colors that the fabric were printed with. You can use them to find matching colors!