Place superfine sugar in a small bowl and, starting with a small amount, stir in the petal dust and powdered or paste food coloring. Pour an egg white into another small bowl and beat it lightly with a whisk or fork. If using roses, peel off any outer petals that are not in perfect shape. If the roses are too tightly closed, tease open the petals by blowing on them and probing them gently with the blunt end of a wooden skewer.
Dip the flower or leaf face down into the egg white. Use the paintbrush to remove all but a thin coating, making sure that the underside of the petals or leaf is coated, too. Hold the flower or leaf over the sugar bowl and, with a small spoon, spoon the sugar over the petals or leaf to coat them evenly on all sides.