When I work on quilts or quilted pieces that are too fragile to handle a great deal, I use two online tools to help me figure out which colors to use for repairs.
The first is a color palette generator from DeGraeve.com. I upload a photo of my project and then generate a palette of colors for the piece. I cut and paste each square onto a photoshop canvas, enlarge them, and print out my own custom swatches to take to the fabric store (first I hold them up to the repar piece to make sure the colors match, because printers and photoshop programs can subtly alter colors during the transfer process.)
Sometimes I like to use a complimentary color, or one with a similar value, which is when I use DeGraeve.com's color picker generator. After I enter the code for the color of the patch I need to repair, it generates a nice variety of alternative colors, from which I can cut-n-paste and print some swatches. Occasionally I'll enlarge the color enough to cut a paper version of the repair patch, which I'll place on the piece to get an idea of how harmonious the color will be before I commit to fabric.
Here are screen shots of both generators in action:
The color palette generator also comes in handy when I see a quilt with colors I love but a pattern I hate; I'll upload a photo to get the color values to take with me to the store.