Doink, my experience has been the same with the lighter colors, specifically the tan, grey, and the yellow to an extent. I have one of the first run White receivers, and I ended up texture coating it black for that exact reason. I tried a few different cleaning products, and they all seemed to work well, I'm just a dirty guy and I beat on my gear pretty hard.
I've got one of the limited edition glow-in-the-dark receivers with matching 20" handguards, you can bet it stays in the safe unless I scour my hands first!
akconvert, I've got at least 4 Cav-15s, maybe 5, built in different configurations. I've been [b]very[/b] happy with their products as a whole, and they've got probably the best warranty and customer service in the industry. I've used their post sample full autos for fun and for competition, and they've worked at least as well for me as any others.
Their new receiver should be available very soon, they're in the final stages of finishing the mold.