Frost the cupcakes with the buttercream. Create a dip in the center of the cupcake about 3/4-inch across. Adhere the coconut grass to the outer edge of the cupcakes.
For the petals: Place the egg yolk-tinted frosting in a pastry bag with a #70 leaf tip.
Hold the bag at a 45 degree angle with the tip lightly touching the surface just inside the dip in the cupcake (with the wide part parallel to the surface). Squeeze hard to build up the base and simultaneously lift the tip slightly. Relax pressure as you pull the tip toward you, drawing the petal to a point near the edge of the cupcake. Stop squeezing and lift the tip away.
Repeat all the way around the cupcake, slightly overlapping the petals. Repeat with remaining cupcakes.
Fold a piece of paper in half and place the chocolate sprinkles in the center. Using the folded corner as a guide, pour the sprinkles into the hole in the middle of the cupcake. Repeat with remaining cupcakes.
Preheat the oven to 250 degrees F. Line a baking sheet with parchment paper and top with a rack.
Toss the coconut with the green and yellow food coloring, depending on desired color. Sprinkle with water if necessary to disperse the color. Drain. Spread evenly on the rack, place in the oven and bake until dry, 10 to 15 minutes. Makes 2 cups.