Another insider look...

In the early stages, the design of the game included as a main feature the necessity of uncovering the mazes by visiting all of their corners. This was conceptually and visually cool, but terrible from a gameplay point of view, as it made the game hard and destroyed completely the strategic aspect of the game (at the beginning of a level, being able to see the whole maze, with its bonuses and maluses, allows the player to choose the best strategy to complete the level while amassing as many points and extra lives as possible).
Therefore, the idea got replaced with a much milder one, according to which the unvisited areas of the mazes were simply darker. That was nice to the eye, but it did not really add much.
So, also the second idea got canned and the two bitplanes that had been reserved for it were used to double to the number of background colors and for the transparecy-based effects that provide feedback when a gem/bonus/malus gets picked up.