Chocolate. The other two have too much contrast, they're too 'primary'. It definitely needs a subdues=d background because the art has more than enough action and 'pop'.
Definitely the brown. The other colors are way too bright, and like Jackpot777 said, those colors would detract from the design. The darker color makes it pop out just right. Love the design, very clever!
Great design, by the way.
Finally it seems that the brown background will be the best option. I'll get to work on it.
I forgot, the title of the design is "Kill Milk", obviously :P
btw amazing stuff. I love this. Violent cracker is my hero.
Some changes in the design on brown background. What you think?