Skin Colour in "Northern" Regions of The earth
Melanin reduces the skin's ability to make Vit D due to reduced UV penetration. This is a disadvantage in northern areas where people get less sun light, and also need to wear heavy cloths to keep warm which adds to the problem. People in Sunny regions have the opposite need as there is more than enough UV light exposure, but they run the risk of developing skin cancer, which is why having melanin provides an evolutionary advantage in these regions.

Interestingly, in the Arabian Gulf region, there is currently an epidemic of Vit D deficiency especially among those with dark skin. Although there is plenty of sun light, people tend to avoid the harsh sun almost completely by commuting strictly in cars and avoiding walking outdoors during the day. Car windows (and glass in general) are opaque to UV rays, therefor reducing the skin's ability to produce Vit D, and forcing many locals who adopt such lifestyle to take oral Vit D supplements.

Here is a link with some references to confirm the medical link between Vit D deficiency and lack of sunlight exposure:

https://www.webmd.com/food-recipes/qa/how-can-dark-skin-lead-to-vitamin-d-deficiency