Since I had color indexing data already created for a color search feature to hand (see other portfolio item here), I used it for color filtering on the Image Conscious product archive page.
There are 8-12 colors stored per product in a custom table, linked to the product posts by SKU. I had to work a bit to correlate all/any possible RGB color to appear as one of a standard, more limited color choice palette for use in the filter sidebar.
See the live version on the Image Conscious web site (scroll down past the other product filters in the sidebar).