Would depend on the art style, but I would just leave the skin without anything to indicate tone. In some people their ethnicity will be evident any way, and in some not, but unless you have a natural way to indicate it, less can be more.
But yeah, I agree with the others that you can use other art methods (hatching, lines, dots) if you're able to make that look appealing. But it could be a great challenge to work on one's ability to draw different facial features instead of focusing on tone as an indicator.