And yet human!Luna has practically always been drawn with darker skin because of her dark blue fur with human!Celestia having pale skin because of her pearly white coat.

Conventional wisdom among fanartists seems to be going of of the fact that since Celestia has a light-colored coat and Luna is dark, a humanized Celestia would be white and human Luna would have dark skin. I don't really agree with that though, it would make far more sense like you said to have a pale night princess and a tan sun princess.