The best way to decorate or paint French macarons after baking is to thin gel food coloring with a few drops of vodka or alcohol-based baking extract. The alcohol encourages quick-drying, so it doesn't harm the shells. You can even splatter piped macaron batter with food coloring thinned this way. It's how the pastry chefs at Bouchon (where I once worked) create the popular speckled French macarons in the springtime.