Strategically applied icing turns basic oval sugar cookies into marbleized masterpieces. Download templates one and two for decorating instructions.
Sift flour, baking powder, and salt into a bowl.
Put butter and sugar in the bowl of an electric mixer fitted with the paddle attachment. Mix on medium speed until pale and fluffy. Mix in eggs and lemon juice. Reduce speed to low. Gradually mix in flour mixture. Divide dough in half; flatten each half into a disk. Wrap each in plastic. Refrigerate until firm, at least 1 hour or overnight.
Preheat oven to 325 degrees, with racks in upper and lower thirds. Let one disk of dough stand at room temperature just until soft enough to roll, about 10 minutes. Roll out dough between two pieces of plastic wrap to 1/4 inch thick. Remove top layer of plastic wrap. Cut out cookies with a 6-inch fluted egg-shaped cookie cutter. Transfer cookie dough on plastic wrap to a baking sheet. Transfer baking sheet to freezer; freeze until very firm, about 15 minutes. Remove baking sheet from freezer and transfer shapes to baking sheets lined with nonstick baking mats. Roll out scraps, and repeat. Repeat with remaining disk of dough.
Bake, switching positions of sheets and rotating halfway through, until edges turn golden, 15 to 18 minutes. Let cool on sheets on wire racks.
Divide royal icing evenly among three small bowls. Color each portion of icing with desired shade of food coloring, stirring with a toothpick to combine. Transfer each color of icing to a separate small squirt bottle. Using the colored icing, decorate according to template instructions.