Until this week I have only worked with white fondant and the cornstarch on the rolling mat didn't bother it. But I'm wondering what you do about cornstarch with brightly colored fondant? Even when I tried to just keep it on the bottom of the fondant, everything I touched got the white on it. Any suggestions?
Dust it off with a clean pastry brush. If any cornstarch remains, paint it with vodka. The vodka evaporates, leaving no taste behind, but gets rid of the cornstarch.
Ditto to what Texas_Rose said. You can also use a tiny amount of shortening and then gently rub it off afterwards as well.
I rub shortening all over the cake and wipe it off with a peper towel.