Doors

A door, is, well, a door. That's it. That is literally it.

Doors

 * Blue doors are "opened" with any type of beam. And by open I mean open with brute force, even though nobody else in the series uses the doors like that, and probably have some kind of keycard that makes it open or something, but Samus couldn't care less about how to do anything properly.
 * Grey doors, sometimes known as "Metal Doors," operate by feeding on the spiritual energy of the enemies in the room. The doors cannot be destroyed without defeating the enemies first, and the enemies cannot be defeated without destroying the door first, creating a logical paradox that causes the game to crash every time you enter a room with a gray door.
 * Red doors are opened literally blown to pieces by Samus using missiles. In Metroid (NES) and Super Metroid, Samus uses 5x as many missiles than any other game.
 * Green doors are the same as above, but "require" even more heavy explosives.