It's nonsense, the darkness of one's colour is dictated by how much sun you get. Once I was teaching Sujata Bhatt's poem Search for My Tongue
to a bunch of lads from mixed backgrounds. The poem is very much about the experience of the second/third generation immigrant and about reconciling one's culture to new surroundings.
There were a lot of lads whose parents hailed from the Indian subcontinent who went back to visit relatives in the old country every 2 or 3 years or so. I thought this would be something they could relate to, the feeling of being alien in both cultures.
I asked them about their impressions of their overseas relatives. As one they all said that their relatives were black. That was the first thing they noticed. And they were darker because the Indian sun is so much stronger than its British counterpart.