Can a variable of physical traits like height, weight, complexion be changed in outer space? Maybe by gravity or by something else that determines these traits (like the UV rays of the sun that determines the production of melanin in the body)? As UV ray presence is not constant in space, would we expect a person with dark complexion to get fairer after living for a sufficient amount of time in outer space where UV rays vary compared to Earth?
There is a Wikipedia article dedicated to the effects of spaceflight on the human body. Some of this is about launch and other aspects outside "prolonged space flight". Still, you might want to see that, but I'll mention a few things below.
For some time, it has been known that long duration space flight leads to spinal extension, making the person taller. However, this effect is reversed after the space traveler returns to Earth. For more, see this space.com article.
Spending an extended time in microgravity leads to deteriorated bones and muscles. This reduces body mass (weight).
There are many other effects, including: redistribution of bodily fluids, changes in eye sight, atrophying of the heart, reduction in aerobic capacity, brain reposutioning within the skull, sleep problems, etc.
I am unaware of any effect on skin complexion (except for the potential of sunburn, which I would expect to be temporary).