Heat oven to 350ºF (325ºF for dark or nonstick pan). Make and bake cake as directed on box for 13x9-inch pan. Cool 10 minutes. Run knife around sides of pan to loosen cake; remove from pan to cooling rack. Cool completely, about 1 hour.
Cover large flat tray or piece of cardboard with foil or plastic food wrap. Place cake on center of tray.
Spoon frosting into large bowl. Stir in food color until well blended. Reserve 1/2 cup frosting. Frost cake with remaining frosting. Spread outside of ice cream cones with reserved frosting. Arrange cones with pointed ends up on one corner of cake for trees. Decorate each tree with 12 to 14 marshmallow bits.
Arrange 2 rows of marshmallow bits from trees to within 4 inches of other short end of cake for walking path. At end of walking path, create pot of gold, using candies and baking cup. Sprinkle green sugar around top edges of cake. Store cake loosely covered.