A full set of doors, from left to right: Red, Green, Blue, Yellow
There are four colored doors in Dr. Lunatic: Red, Green, Blue and Yellow/null. The doors disappear when approached with the corresponding Keys. Red, Green and Blue keys remain persistent and can open as many doors as necessary, but yellow keys are used up after one use.
The Item Editor can create other kinds of doors, but this requires extra work on the world builder's part, as they need to make the specials that recognize the key(s). It's also possible to create "global doors" that can be opened from a key collected in another level.