Looks like there were several waves of “original” humans into Europe so its a difficult question to answer. They have theorized the early hunter gatherer population had dark skin and blue eyes.
I could see that the darkest babies in a seasonally low sunlight, cloudy latitude and climate not getting enough Vitamin D and therefore not thriving or even dying.
So lighter skin would be a life long advantage in such places. The lighter skin individuals would at least maximize their Vitamin D production over the darker skinned ones and pass their genes on by producing children—or more children over a longer life. How many generations would that take? I would think less than 100.
So there’s my take on skin color. Still, Eskimos are far darker than Laplanders. Why is that? Both are at high latitudes and similar climates. Maybe Eskimos get far more Vitamin D from fish, seals, whales and polar bears in their diet?