How do they prevent everyone on starships from being "pasty white"? How do people get Vitamin D? What about getting a tan? Different people have different sunlight tolerances. How do they ensure that a more fair-skinned person doesn't get burned, which can easily happen even with normal sunlight? Any explanations?
The question contains three basic logic flaws.
There is the assumption that people spend most of their time on starships. While there may be some people who live their entire lives in space, most people live the majority of their lives on planets. They are born and raised on planets. As Kirk explained in Star Trek IV, he was from Iowa, he only worked in space. Picard, as we know, is from France.
The second problem is the assumption that, in space, people would not be availing themselves of artificial sunlight. Seasonal Affected Disorder is no joke. Neither is vitamin D deficiency. All spaceships would have artificial sunlight available for medical reasons. Also, humans just enjoy the feel of sunlight on our skin. I have no doubt that beach resorts are commonly used hologram programs.
Finally, regardless of the first two points, some aspects of skin tone are hereditary and it would take a considerable amount of time (millennia?) before living in space made any difference in this regard.