I obviously can´t speak for WW2 sub crews, but the modern day german infantry uses blue instead of red. Preserves the night vision as well as red does and can´t be seen as far. Something about the ambient light having blueish components, i think. (I had to learn that, but, well, i forgot the exact reason.)
Actually the standard issue flashlight has four colour filters: Red, yellow and blue, so you can use it for colour coded signalling.
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