Preheat oven to 325° F. Line rimmed baking sheets with parchment paper or silicon mats; set aside.
In a medium bowl sift together the almond flour and confectioners' sugar; set aside.
In a large bowl, beat egg whites on high speed until foamy. Add cream of tartar; beat on high to combine. Slowly beat in granulated sugar, 1 tablespoon at a time. Add vanilla extract. Continue beating on high until stiff peaks form.
Add green food coloring. Beat on high until desired color is achieved.
Sift almond flour/confectioners' sugar mixture over meringue. Gently fold mixture together. At first, the mixture will be thick, but will thin out as the folding continues. To test for the proper consistency, drop a small amount of the batter and count to 10. The batter is ready if the dropped batter dissolves after 10 seconds.*
Transfer batter to pastry bag fitted with a #12 or A1 round tip. Pipe 1-½ inch circles on parchment or silicon lined baking sheets, leaving an inch between circles. Tap baking sheets firmly on counter several times to remove any air bubbles. Allow the macaron shells to dry and form a skin for about 30 minutes. To see if the shells are ready to be baked, lightly touch one. If the batter does not stick to your finger, it is ready.
Bake shells in preheated 325° F oven for 10 to 12 minutes, rotating pan halfway through. The macaron shells are done if the bottom does not stick when removed from pan.
Remove from oven. Allow to cool on the pan for 15 minutes. Transfer to wire rack to cool completely.
Repeat this entire process for the purple macaron shells.
While the macarons are cooling, prepare the lemon buttercream. In a medium mixing bowl, beat the butter on high until light and creamy. Add confectioner's sugar, cream, lemon juice, vanilla extract, and salt. Beat on high until light and fluffy. Add yellow food coloring; beat until well combined.
Transfer lemon buttercream to pastry bag fitted with #7 round tip. Pipe buttercream on green macaron shell; top with purple macaron shell.
Store filled macarons in airtight container in the refrigerator for up to a week. The macarons can also be stored in an airtight container in the freezer.
Yield: 36 Mardi Gras Macarons when the macaron shell recipe is repeated.
*Note: The consistency of the macaron shell batter is critical. If it's too thick, the macaron shells will crack during baking; if it's too thin, the batter will spread too much and the macaron shells will not develop the characteristic feet.