Door

Door
A door is a pathway which leads ;;Samus]] into new rooms. They have appeared in every Metroid game to date except Metroid Prime Pinball Doors almost always feature low-level energy shields that can be disrupted by weapon fire. The most common type of door being a blue door, which can be opened with any type of weapon. Almost as commonplace is the red door. In the original Metroid,and Super Metroid they had to be opened with 5 missiles, and in all preceding games, one missile. Other doors include Power-Bomb doors, which are yellow, Super Missile doors, which are green, and Dark,Light, and Annihilator doors, which open with their respective beams. In Metroid Fusion, the doors weren't opened with weapons. They were opened by unlocking their color security level. Afterward they could be opened with any weapon.

Trivia
In the 3D Metroid games, the doors acted as hidden loading screens, which is why some took so long to open.