All People Quilt.com (the website of American Patchwork & Quilting magazine) has a very educational piece on using fabrics from your stash together. It is called Stash Rx and you can find it here.
The article has advice and guidance from Weeks Ringle, a quilter who very effectively mixes fabrics from different collections and different styles in her quilts. And she includes some color advice I've been told before in the context of interior decorating, that is, to have one color that is more dominant, along with a secondary color and one or more accents that are used to a lesser extent than the secondary color. “Equal proportions of colors makes for a less interesting mix overall,” Weeks says.
I have a hard time remembering this guideline but now that I've seen it in action, it might stay with me. The story takes Jennifer Keltner through the process of selecting some additonal fabrics to round out the five from her stash, then shows the finished quilt that she made from her selections.
If you sometimes have a hard time deciding on the best mix of fabrics to use in a quilt, click over to All People Quilt.com and read the story and look at the photos that go with it. I think you will be glad you did.