GIS now lets you filter searches by colour. I’m sure this is new, there’s not web UI for it yet, but it is available if you tweak the URL. Here’s what I used to get that screenshot:

http://images.google.com/images?imgcolor=red,yellow&q=site%3Alast.fm+inurl%3A174

Wondering what the query parameters do?

• imgcolor - Takes a comma-separated list of colours. I’m not sure the full list, but try the obvious ones, named/html colours is a good place to start.
• site/inurl is intended to limit the results to album covers from Last.fm. The specific 174 value is the dimension of our album images, this filters out most artist/user images.

So with a pretty simple search query you can find red and yellow album covers. Oh, and for programatic use the result links to the album page.

That’s damn powerful. I can’t wait to see other cunning use of this.

Edit: As @kaichanvong notes, programatic access would be nice. Unfortunately the Google Image Search API doesn’t support the imgcolor param yet. I’m sure it will, but it’s annoying for now.