Hamtaro: Rainbow Rescue [GBA]
Press Up, Down(2), Left, Right at the 'Press Start' screen.
Secret Coloring Picture at the Clubhouse
Stand in front of the blackboard in the Clubhouse and press the A button. After reading the message, press the L, R, A, B buttons in this order and you'll receive a secret coloring picture.
Getting the secret picture 'Too Much Good Stuff'
Go to Hamstarr Manor. Behind Seed Pig-Out (the one with Mean Oxnard) there are three hamsters sitting behind a table. Talk to the middle hamster ONCE. Then, press the following buttons in order: B, L, A, B, A, R. You will receive the picture 'Too Much Good Stuff'.