Use sugar cookie recipe to make sugar cookies, and cut into stars. If you don't want to make sugar cookies, you could use store bought dough and decorate with the royal frosting. I much prefer the homemade though!
Once cakes have cooled completely, level tops of all cakes. Use a cake leveler or knife to cut 8" red cake, 8" white cake in half (you will then have 4 thin 8" cakes).
Put an empty 6" pan in the center of the blue cake. Use a knife to gently press down around the pan so that you have an outline of the 6" pan. Remove the pan and cut through the cake along the outline so that you can remove the center of the blue cake (you will not need the center you're cutting out, so you can use it as a snack while you decorate the rest of the cake).
Place one the 8" red layers on a 10" cake board. Use red frosting to pipe a border along the outside edge of the top of the cake. Fill inside the border with half of the raspberry filling.
Place an 8" white layer on top of the raspberry filling. Cover layer with white frosting (I used a pasty bag with Wilton tip #12 for this).
Repeat with next red layer/remaining raspberry filling. Place final white layer on top of this, but do not cover with white frosting. Place red 6" cake on center of white layer. With a Wilton tip #12, use blue frosting to pipe around the red layer. Then place blue layer on top of blue frosting.
Place final 6" white layer inside of blue layer. For whichever layer is shorter, use corresponding color frosting to raise height to that of taller layer. My white was taller than my blue, so I used blue frosting to pipe on top of the blue cake so that the frosting was the same height as the white cake.
Divide your white, blue, and red frostings into three pastry bags. Use a Wilton #12, or similar round tip for all of the bags. Start at the bottom of the cake and pipe around the red layer with red frosting, the white layer with the white frosting, etc. Use the blue to color around the blue layer at the top. For the top of the cake pipe around the blue with blue and the middle white with white.
Use Ateco frosting smoother, or something similar, to go around outside of cake. Just holding the smoother while going around the cake will cause the colors to blend together beautifully. Wipe off the smoother and use it for the top of the cake as well.
Use the white frosting with the Wilton #12 tip to pipe 8 evenly spaced mounds on the top of the cake. Place a star cookie into each mound. You can also place mounds in the center for a larger cookie as well.
Change the frosting tip to a Wilton #21, or similar star tip, and pipe stars along the bottom outside of the cake.
Use sprinkles to decorate further as desired!