Heat oven to 350°F. On a clean surface, knead flour into dough. On a floured surface, roll dough to 1-inch thickness. Cut out heart-shaped cookies, place on a baking sheet and bake 20 to 24 minutes, until lightly browned. Cool on baking sheet 5 minutes, then transfer to cooling rack and cool completely. Re-roll scraps, cut and bake more cookies.
Make Royal Icing. Divide icing between 4 bowls. Tint icing pale pink, medium pink, and dark pink. Leave 1 bowl white. Thin icing with 1 tsp. water per bowl until it's the consistency of heavy cream.
Using the tip of your index finger, spread one color icing over half of a heart. Spread a different color icing over the other side of the heart. Using the tip of a toothpick, drag the icing back and forth over the dividing line to gently swirl the colors together. Sprinkle crystal sugar on edge of cookie if desired. Repeat with remaining cookies and icings. To create polka dots, dip a toothpick in icing and then place drops on iced cookie.
In a large bowl, with mixer on low, beat together confectioners' sugar, powdered egg whites, and water until well combined. Increase mixer speed to high, and continue beating for 5 minutes, until stiff peaks form.