The other factor might be not knowing how to apply terms like dark and light in that context - as Azizi said there's a very wide range of variation, most people on that range would be lighter than some and darker than others.

"She's darker than you" or "she's lighter than you" would be one way to solve that one, but I think some people might take that as over personal.

And I know I'm useless at matching shades of colours when I'm out with my wife shopping, and I doubt I'd be much better when it came to finer shades of skin differences.