I bought my second Apple Watch with a white sports band. Recently I noticed strange staining / discoloration on the strap:

It didn’t occur to me until today, exactly a day before the warranty expires, that Apple had a similar problem a long time ago: White MacBooks suffered from palm rest discoloration, and Apple fixed that free of charge! (my dad’s had the issue, but mine didn’t).

So I took it to the store… not covered.

Then I remembered the tip floating around back then for the MacBook issue: use Mr. Clean Magic Eraser! I’m happy to report that it truly is magical and it worked like a charm. Hopefully this tip helps someone around the internet one day.