Roll some gum paste out to 1/4 inch thick onto wax paper with a rolling pin. If the rolling pin sticks to the paste, dust it with a little powdered sugar. The sugar will act like flour does when you're working bread dough.
Cut out as many leaves as you need from the gum paste with different shaped cookie cutters. Try maple leaves, oak leaves, apple tree leaves and any other shapes you can find. Choose your cookie cutters first, then find rubber stamps the same size and leaf type as the cutters.
Gently remove each gum paste leaf to a clean piece of wax paper. Press the corresponding rubber stamp into each leaf. The rubber stamps must be new, never used. Clean them with a little mild dish soap and rinse thoroughly before stamping.
Crumple up some aluminum foil and smooth it out just a bit. It should still have plenty of ridges and ripples in it. Lay the gum paste leaves on the foil to dry. They'll dry in curls and ripples this way, just like real dried leaves.
Mix a tablespoon of grain alcohol with each of your powdered colors. Paint the leaves with a clean paintbrush, brushing a base color over each leaf. Vary the colors, using equal amounts of red, yellow, orange and green. Let the paint dry for an hour.
Brush over each leaf with a little brown to make them look aged. Let the leaves dry and cure overnight. Decorate pastries, cupcakes, sheet cakes and even pies with these leaves.