Preheat oven to temperature indicated on box (325 F for me). Prepare cake batter according to package directions.
Reserve approximately ½ cup of batter. Add food coloring to remaining batter and stir until the batter is evenly colored.
Brush pan with pan release (or spray with a commercial baking release spray). Pour in the green batter. Tap the pan on the counter 3 or 4 times to release any air bubbles.
Pour the reserved white batter over top and use an offset spatula to spread to the edges of the pan.
Bake according to package directions. Let cake cool in pan for 10 minutes before turning out and allowing to cool on wire rack.
Stir together the powdered sugar, butter, almond extract and milk to form a thin, slightly runny glaze. Pour a small amount over the tops of the trees and use a pastry brush to brush it over the sides.
While the glaze is still wet, sprinkle with sugar pearls or sprinkles. Dust lightly with powdered sugar.