To decorate the cookies, you'll need to dye your royal icing. Split the icing into 3rds and dye one bowl red, one bowl blue and leave the 3rd white. Use 8-10 second icing to outline and fill your cookies.
Fill your pastry bags with the icing. Cut the very tip of the pastry bag. The hole should be very small - small enough that you have to apply some pressure to the bag to release the frosting, but not too small that it's a struggle to frost the cookies.
Draw the outline of half of the star in red to create a barrier. Then, fill in the cookie with the red icing. Immediately, take the white icing and draw stripes. This will create a wet-on-wet technique and the white stripe will bleed into the white so it's a smooth surface.
Then, draw the outline of the other half of the star with the blue icing. Fill in that half with with blue icing. Immediately, take the white icing and quickly squeeze white dots over the blue icing.
Let dry for 4 hours before serving.