Red light is used at night for another reason: it doesn't flood through the periscope giving away your position.
Like the grill in your microwave preventing microwaves leaking out, the size of the aperture of the periscope pretty much prevents the red light flooding out of the scope, so at night you don't give away your position.
If you use white light, then the periscope would be visible at a distance, and could give away the presence/position of the boat.
You could use another option: get depth-charged "enough" that your bulbs fail and you have no internal lighting... Thankfully we have hand-held torches!
I always go to red lighting when attacking, and all night when it's dark. It also acts as a clock so you know whether it is day or night.
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