So, I have 3 ideas.
1: The door opens slightly each time it is shot, and progressively becomes blue-er. The problem here is that the missile explosion will probably obscure most of it, and if moved behind, may look awkward.
2: Special doors have a lock around them with a symbol indicating what weapon will break them, similarly to the different block types found throughout Metroid games. Due to the size of the door, however, making the symbols clear without looking awkward would be difficult.
3: The door opens with 1 missile.
1: The door opens slightly each time it is shot, and progressively becomes blue-er. The problem here is that the missile explosion will probably obscure most of it, and if moved behind, may look awkward.
2: Special doors have a lock around them with a symbol indicating what weapon will break them, similarly to the different block types found throughout Metroid games. Due to the size of the door, however, making the symbols clear without looking awkward would be difficult.
3: The door opens with 1 missile.