I think a shop by color function would be unique and popular. Plus it would index items by color which could prove to be popular with SEO as that would be a new area of indexing content not widely used yet. This idea could be as simple as adding a few extra columns in the table for indexing the items add storing a RGB hexadecimal color. The admin could use a color picker when adding the items to help select a color corresponding to the item.
The php to search for a color could be as simple as
SELECT DISTINCT item_id
WHERE color_r BETWEEN '$colorR'-$margin AND '$colorR'+$margin
AND color_g BETWEEN '$colorG'-$margin AND '$colorG'+$margin
AND color_b BETWEEN '$colorB'-$margin AND '$colorB'+$margin
FYI: $margin == a margin of error to help widen the search criteria.
From a developer stand point this could be used injunction with other APIs like adobes kuler or colourlover.com which could provide quality color swatches to use as a searching criteria. The user would pick a swatch and know all the colors match and then they could look items corresponding to the different colors of the swatch providing the perfect way to look for a wardrobe or outfit.
I have been playing with this idea as a side web service for aggregating popular ecommerce site by indexing content by color.
Any ways thats just my 2 cents.