After watching a lot of sub movies, I've noticed the red light that replaces the normal white light, when the sub is in a war-time action.
1) It's all "Hollywood", and not true IRL.
2) It's in fact used on real subs.
If "2", then: Now... I can understand that you would use red light on the bridge etc. on a surface ship, as the red light is harder to spot on a distance, than white light. (The eye is less sencitive to red light wavelength)
If you then look at a sub in similar action, why in the world would you use red light in it too? You're in a sealed steel container, below the surface of the water, not one single window anywhere.. Is it because the scope (Old type with optical connection to outside world) MIGHT "leak" light from within the sub, when the scope is up? I cannot imagine this is the cause, as it would be pretty easy to enforce "one way only" view in a scope. Simply add a one way mirror to the optics, or some other and better restriction.
Second reason I doubt this, is the fact that the scope is only up for a few seconds, AND someone has his eyes glued to the inside end, blocking the lenses.