regarding use of red light in the evenings, I thought the main reason (and I cannot recall where I read this so am happy to admit this could well be wrong) for using red light was because it had less impact on your 'night vision' than white light. It takes the human eye quite some time to adjust to low light conditions which is why if we step out of a well lit room into a dark night it's very hard at first to see anything, but our eyes adjust if we stay out in the dark until we can see at least a little (save for in pitch darkness - the absence of any light, of course).
Once our eyes have adjusted to low light conditions, even a brief amount of white light (normal elecrtic lighting) will usually mean we have to go through the whole adjustment process again when back in the dark - only takes a few minutes, but thats a long time if your looking for enemy ships
Switching to red light in the evenings therefore makes it easier for the kaleun and crew to preserve their 'night vision' and spot things on the surface in the dark. In fact, I'm sure in one evening scene in Das Boot you can even see the Kaleun wearing darkened goggles on his head - which I assumed (if an accurate historical reflection) was to help him preserve maximum night vision.
Like I said, could be wrong. However, am pretty sure the above is accurate.