Press START during gameplay.

Input BLUE, BLUE, BLUE, YELLOW, BLUE to add 1000 credits.

Input BLUE, BLUE, BLUE, YELLOW, YELLOW to add 2000 more credits.

Input BLUE, BLUE, BLUE, YELLOW, BLUE, YELLOW to add 5000 credits.

Input BLUE, YELLOW, YELLOW, YELLOW, BLUE, BLUE, YELLOW, to add 10.000 credits.

The above inputs may be used once per game (you can do it again on a New Game Plus).