I made this for a contest, and figured I'd post it. Haven't quite fully tested it, but it should work well.
The idea comes from that game "Mastermind" where you attempt to determine a precise combination. You have to determine the proper colors and the place they go in with five rounds, otherwise, you lose! This game is modifiable if you know your way around it, but for now, the combination should work fine. Also, spawning might not be working correctly, if it doesn't, just go into /gamemode 1 and go up into the smallest sandstone room with no redstone beneath it (warning though- the combination is on top of the room, no peeking!).