Prepare buttercream according to recipe.
Set aside a small amount of uncolored frosting in each of two bowls. One will be left untinted for the white on the Santa hat; one will be tinted yellow for the Grinch eyes.
Divide the remaining buttercream evenly between two large bowls. One will be tinted green for the Grinch face; one will be tinted red for the Santa hat.
Add food coloring gel to the bowls as described above. Fill piping bags fitted with tips as follows: the yellow and black (store bought frosting) should have small round tips, the white should have a small star tip, and the green and red should have large round tips (or just snip off the piping bag to make an opening approximately 1/2-inch in diameter.)
Pipe a ring of green frosting on the cupcake around the edges of the cupcake and going straight up, each ring slightly smaller, about 3 or 4 rings to create the face of the Grinch, leaving a hole inside the tunnel of frosting.
Fill the frosting tunnel with M&M's.
Pipe the red frosting in circles on top of the green frosting/candy to create the Santa hat.
Add a small white mound of frosting on the top of each Santa hat and small stars around the base of the hat for the white fluff.
Pipe on the yellow eyes to the front of the Grinch face. Add the black eyebrows and pupils.
Store cupcakes in an airtight container until ready to serve.