• Prepare a piping bag fitted with a large round tip or with the tip of the bag cut off.
• Secure in a piping bag holder or place in a tall drinking glass with the top of the bag folded over. Set aside.

Prepare the Batter:

• Sift together the oat flour and powdered sugar in a small bowl. Set aside.
  63 grams oat flour, 63 grams powdered sugar
• In the bowl of a stand mixer fitted with the whisk attachment, beat the egg whites on medium-high speed. Once the egg white are foamy, add the cream of tartar. Then, slowly add the granulated sugar, one spoonful at a time.
  50 grams egg whites, ⅛ teaspoon cream of tartar, 45 grams sugar
• Add the vanilla and food coloring and continue to beat until the meringue has formed stiff peaks. For a dark green shell, add green food gel and a tiny amount of black on a toothpick.
  ½ teaspoon vanilla, green and black food coloring gel
  
• Remove the bowl from the mixer and carefully fold ⅓ of the flour mixture into the meringue. Then add the rest of the flour mixture. Continue folding until just before the figure 8 stage. It is better to slightly undermix than to overmix.
• Transfer the batter to the prepared piping bag.
  
• Pipe 1-inch circles onto the macaron mat or parchment with 1-inch circles drawn on the under side of the paper.
  
• Slam the tray on the counter several times to cause air bubbles to rise to the surface. Use a toothpick to pop any large bubbles and smooth batter over the hole.
• Allow the batter to sit at room temperature for 40 minutes to dry. Meanwhile, preheat the oven to 300°F.

Bake the Macaron Shells:

• Bake the macarons for 12-15 minutes, rotating once after 7 minutes.
• Remove the cookies from the oven and allow them to cool completely on the mat before removing.
  
• To remove the shells, push up from underneath the mat to loosen them. Properly baked shells will pop off easily and have a smooth, shiny bottom.

Prepare the Buttercream:

• Divide the buttercream equally among 6 bowls. Color the portions red, orange, yellow, green, blue and purple. Use both red and yellow gel to make orange, yellow and blue gel to make green, and red and blue gel to make purple.
  
• Fill each of 6 disposable piping bags with a single color of buttercream. Snip off the tip of each bag to create a ½-inch diameter opening and push the buttercream down to the opening of the bag, making sure to get rid of air pockets.
  
• Place a large rectangle of plastic wrap on a clean, flat surface. Pipe an 8-inch line of each color buttercream in rainbow order.
  
• Carefully roll the plastic wrap into a log so the colors form a circle with no space in the middle. Twist both ends of the plastic wrap to secure.
  
• Cut off one end of the plastic wrap close to the buttercream, and place the log inside a piping bag fitted with a large star tip.
  
• Press the buttercream evenly on all sides of the bag until it comes out the tip and pipe some onto a paper towel to make sure all the colors are exiting the tip.

Assemble the Macarons:

• Because the macarons will be sandwiched, pair up the shells by size. You want the top and bottom of the sandwich cookie to match as closely as possible.
• Place one shell flat side up and pipe a mound of buttercream into the center.
  
• Carefully hold the matching shell by the edges and sandwich the macaron, twisting gently as you press down. Repeat with the remaining pairs.

Storage:

• Store the assembled cookies in an airtight container at room temperature or in the refrigerator for several days.
• If refrigerated, allow the cookies to come to room temperature before serving.