Metroid: Father Hunt

[Chris2Balls [:B]]

Interesting! Your first idea could work pretty well if it stayed blue-ish for a short amount of time before fading back to red; it'd encourage the players to carry on shooting missiles before it resets completely to red. For your second idea, you could make an enemy or creature or something use some kind of missile to tell the player that's how you open the door. That being said, you could also have a sound cue for objects that can be destroyed with missiles. The third idea might be the easiest.