The amount of light that is passed in to the trailer. You may want a smoked or black for a bedroom if you wish to reduce the amount of light.
I have white on all of mine, and by themselves reduce some light. I added 'heavy duty' vent plastics on two that needed to be replaced, which reduced light entry even more. I have foam/foil pillows for the vents to insulate and eliminate any light entry, so the white covers were the best choice for me.