What exactly is Northern Daylight ? And what does it have to do with better color perception ?

Well, I'm guessing here, but in the Northern hemisphere north light is skylight. The sun is always a little bit to the south, depending on your latitude and the season. On a clear day you'll have a big diffused light source...but a very blue one. On a cloudy day, the color temperature will not be as cool, but still cooler than direct sun.