How much world-building have you done for your story-verse? You could probably decide on skin tones and your world's ethnic differences if you have decided on things like climate, history (like conquering forces moving in, trade hubs, tapped out aquifers prompting a mass exodus, etc.), lifestyles, technology level, and/or what on earth they think is "pretty" and a desirable trait, often because it's a status symbol proving a person is rich... like how paleness can be in style to prove one doesn't do outdoor work or how tanness can be in style to prove one has time to sunbathe and take exotic vacations to the Caribbean. Or I dunno. Everyone is horrified of vampires so dark skin is popular because vampires are lily-white bastards.
I hoard skintone color swatches in Photoshop. I think I saved them from a coloring guide long ago and I add new ones from time to time. Other times I just pick whatever from the color picker.
This guide to human types is probably worth a look: part
1,
2,
3. Since there's more to diversity than just skin tones.