A full set of keys: (from left to right, up to down) Red, Green, Blue, Yellow
Dr Lunatic[]
There are four coloured keys in Dr. Lunatic: Red, Green, Blue and Yellow. These match the Doors which have panels of the same colours, except for the 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 one door per key and only a maximum of three yellow keys can be held in the inventory.
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.
Kid Mystic[]
Doors and Keys operate identically as they do in Dr. Lunatic.
Sleepless Hollow[]
Red, Green, Blue and Yellow keys work the same way in Sleepless Hollow, except that they are kept between areas.