I have painted on fondant several times. I have used pure extracts. Many others use vodka. I want to switch to using vodka, extracts dry too quickly.
If it's for a wedding cake, and you have never painted on fondant before, I would suggest a test run. Days before you have to work on your wedding cake, (less stress!)
Grab a scrap of fondant, grab your food only brushes and see what you come up with. I have to 'play', with the brushes on a paper towel and blot off some of the excess to get the food gel's and the extracts to the correct consistency.
Hope this helps!