Source: Martha Stewart's Cupcakes
Starting with 1 drop at a time, add gel-paste food coloring to buttercream. Stir well to combine. Add more food coloring, stirring constantly, until desired color is reached. Dab buttercream on top of pastry nail, and press a 2-inch square of parchment on top. Reserve half the buttercream for frosting tops of cupcakes. Fit pastry bag with coupler, and fill with remaining buttercream. Place tip of coupler in center of parchment square, squeeze pastry bag gently, and pull up slowly to make an acorn-shaped mound (this is the bud).
Fit pastry bag with petal tip. (You can anchor the nail in Styrofoam or a potato with a flat-cut bottom.) Hold tip against the point of the bud, wide end down and narrow end angled in toward center. While turning the nail slowly, pipe a wide, tight strip, enrobing the top of the bud. Turning the nail as you go, pipe 2 arched petals that each reach around half of the bud. Continue turning the nail and piping, making longer, overlapping petals and angling further outward as you go, until you reach the bottom of the rose. Gently slide parchment with rose onto a baking sheet, and refrigerate until firm, at least 20 minutes (or up to 2 days). Repeat to make more roses.
Frost tops of cupcakes with buttercream. Using a small offset spatula, gently place a rose on top of each cupcake, discarding parchment.