Cuthbert Goes Walkabout is a 1983 computer game for the Dragon 32/64, TRS-80 CoCo, Commodore 64 and Atari 8-bit family home computers. Produced by Microdeal, the game features the hero Cuthbert. The game is based on the Konami arcade game Amidar. The player has to guide Cuthbert around a grid-like level of squares. When the player walks all the way around each square it is filled with colour. If all the squares are filled, and Cuthbert successfully avoids the monsters and finishes before the time runs out, the player progresses to the next level. The levels have a green background, except for every third which is white as a sort of bonus level.
Source: Wikipedia, "Cuthbert Goes Walkabout," available under the CC-BY-SA License.