Along with others who posted, at first a true scrappy, totally unplanned color combinations, looks terrible but as the quilt grows the colors seem to just snug up close to eachother and play well together. I love those truly random looks. It also allows me to just build the quilt without sorting through piles, boxes and bins of scraps. Now, if you have a depth of scraps that allows you to focus on one kind of fabric, batiks or reproduction prints for instance, then go ahead and use one kind of fabric in a single quilt. You will see so many good examples of both kinds on the Quilting Board. Half of the challenge is getting over planning every color in the block. For the most part I have found it freeing and the end products are glorious fun to see.
I do from time to time pick a single fabric as sashing to tie all of the beauty of the random color choices together but not always. In the end, do what feels good and right for you.
Like all quilting, it is the adventure of creation that keeps us coming back again and again.