Make the Cookies: Whisk together both flours, the baking powder and salt in a medium bowl; set aside.
In the bowl of your mixer, rub together the sugar and grapefruit zest with your fingertips until the zest is evenly distributed and all of the sugar has been moistened. Add the butter to the bowl and beat on medium speed until the mixture is light and fluffy, about 3 minutes. Add the egg yolks and beat until combined, scraping down the bowl as necessary. Reduce the speed to low and add the flour in three batches, alternating with the grapefruit juice, beginning and ending with the flour. Mix just until the dough comes together. Transfer the dough to a piece of plastic wrap, pat it into a 1-inch disk, wrap in the plastic wrap and refrigerate for at least 30 minutes, or until firm.
Preheat oven to 350 degrees F. Line two baking sheets with parchment paper or a silicone baking mat.
Roll out the dough on a lightly floured surface to about ⅛-inch thick. Use a 2-inch round cutter to cut out as many cookies as possible and transfer them to the prepared baking sheets, spacing them 1-inch apart. Gather together the dough scraps and repeat to get as many rounds as possible. Refrigerate the baking sheets (with the cookies on them) for 15 minutes.
Bake the cookies until just barely golden brown around the edges and still very light in color in the middle, about 12 minutes. Immediately remove the cookies to a wire rack and cool completely before assembling.
Make the Filling: Beat together the butter, powdered sugar and grapefruit zest on medium speed until light and fluffy, about 5 minutes. Add the grapefruit juice and food coloring (if using), increase the speed to medium-high and beat until creamy and fluffy, about 2 minutes.
Assemble the Cookies: Match up the cookies by size, then spread or pipe on about 1 tablespoon of filling onto the flat side of half the cookies. Top with the matching cookie and press down slightly so the filling is pushed to the edges. The cookies can be stored in an airtight container at room temperature for up to 5 days.