There are 4 coloured keys in Dr Lunatic, Red, Green, Blue and Yellow. These match the Doors which have panels of the same colours, except for yellow doors, which have no panel. Red, Green and Blue keys can open as many doors as there are in the level, but yellow keys can only open 1 door per key and the player can only hold 3 at once. In Supreme With Cheese, the Item Editor can create keys of different colours, but that requires extra management on the part of the builder, as they also have to make the corresponding Doors and Specials to recognize the new keys.
Red, Green, Blue and Yellow keys work the same way in Sleepless Hollow, except that they are kept between areas.