by mixing the genes
not at all, it depends on which genes prevail at the respective child. For example, I know several families in which one partner is dark-skinned and the other knows and have a white child and a dark child. There can also be two black or two Latinos a white child or turn two white a black child when that is in the genes.
Often it happens that children see their grandparents similar to their grandparents or great-grandparents.
And that's a good thing, so the variety remains preserved.
probably never because areas with strong sunlight are a darker skin color.
hard to say because that takes a long time
But very long is not so important anyway