Divide the frosting into two bowls, 2/3 of the into one bowl and mixed with green food coloring to make "grass" and 1/3 into a different bowl with yellow food coloring to make "gold".
Place the graham cracker on a work surface, if working with kids, best to use a paper plate with a rim to catch any mess.
Either slather on or pipe on using a "grass" tip or Wilton tip #233 to make the grass.
Place a brownie muffin on one side. Using a round tip or star tip or Wilton tip #21, make the "gold".
Cut 6 of the rainbow Airheads to 3.5 inches. Loop one half sticking it into the "gold and the other half into the "grass.
Surround the rainbow on the grass with 4 mini marshmallows.