Just to add, the mesolithic hunter gatherers of Scandinavia and Europe had dark skin for around 40k years or more after their migration out of Africa. There wasn't a need for them to adapt their melanin to colder climates because they ate a diet rich in vitamin D3, creating a shortcut that doesn't require sun exposure. They consumed fish, the livers of large mammals, and mushrooms. They were also lactose intolerant like their African ancestors. They did have the mutations for light eyes.

It wasn't until migrations to the area during the neolithic period that farming was introduced to this area, and hunter gatherers lived with farming societies for a thousand years or so before the complete adoption of agriculture. These migrations, along with climate change, the loss of hunter gatherer diet, genetic mutations for lactose tolerance, mutations regarding immunity, and sexual selection, led to lighter skin, which just happened in the last 4k years or so. Archeological evidence coupled with DNA show that vitamin D deficiency did become a problem during this transition.

I'm glad you brought up vitamin D deficiency, because it is really a problem, especially for those with darker skin near the equator. They have fewer natural sources of D3, and some cultural and religious practices in these areas lead to women, who need Vitamin D the most for bone health because of estrogen, to suffer from vitamin D deficiency in the highest numbers. The children in these areas are disproportionately affected as well.