Mate in one. White to move. If you're first to solve you win a cookie. =D
I left in the 'sand' BG because I was using it for a game mock-up set in Persia. The tiles could definitely use more variation, and I'll work on it more if the game gets greenlit.
Your right. I think there is really 18 colors, i just missed some stray pixels somewhere...
Edit: Got it down to 23!