Why do the whites of dark-skinned people seem to tinge brown rather then red when they’re tired or have been crying or whatever normally induces red-veins in the whites of the eyes of light-skinned people?
Assuming this is true (I can’t say I ever noticed), a reasonable guess would be that the same melanin that causes dark skin and dark eyes has an effect.
Is this what you’re talking about? If so, it’s a normal variant that is always present in some people, especially those with dark skin. I do not think that it has anything to do with the vasculature of the eye, and am not aware of it being more obvious after crying or when tired.