Ooh! Ooh! I know sir! Ask me! I know!
The rod cells in your eyes (used for night vision) are most sensitive to blue and green light. These higher frequencies cause the night vision system to shut down to avoid being overloaded, and it takes a long time for them to return to full capacity if you look at white light.
I also read somewhere that it's for orientation - when most of the crew rarely see daylight, it provides a psychological way to keep track of the days passing by.
Besides, it's spookier (and therefore cooler) this way.
Moar:
http://en.wikipedia.org/wiki/Purkinje_effect