Correct, red light was used only in the parts of the boat where the relief watch crew was waiting for its shift, getting used to low light conditions. Conning tower and at most, control room. The rest of the Uboat would use normal light, I believe I read somewhere the blue light we see in Das Boot was never used in real life, but added because it made faces even more pale and the atmosphere inside the boat even more claustrophobic as well as helped indicate changes in time/status of the mood (No daylight was visible in the interiors, which means it's hard for the director to give big changes in appearance to the scenary to transmit feelings or sensations).
That said, IIRC some time ago there was a blue light mod around that changed the normal white one for blue in certain parts of the boat.
