Probably already been said but body builders get tans because reflections accentuate curves and reflections are more visible on dark surfaces than pale ones. Body-building competitions are usually lit by spot lights. Imagine a white-as-snow body builder, their muscle definition would be less visible in that lighting context.
On the other hand, if the lighting was highly diffuse, such as under an overcast lighting condition, the pale color would make curves more distinguishable. Roman or Greek statues often find themselves lit in a less direct manner for that reason.
Note that I have never read or watched One Punch Man, I'm not commenting on that, just the body building tans.
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