The long and the short of it, if you can't walk out in the middle of the field and accurately discern red light from green by the age of 17 or later, you probably aren't really safe to fly at night.
but I thought they tested you in the day?
The operating restriction issued for failure is against night flight because you need red/green to know what your traffic situation is.
Why didn't I think of that?
Never knew the reason but I didn't really think about it.
Cheers