We used pre-made frosting, but if you want to make your own vanilla buttercream, this Homemade Vanilla Buttercream Frosting recipe is really good and easy to make and the link is in the notes.
For 12-18 cupcakes, you'll need 3 tubs of vanilla frosting. If you are making 18 you might want to opt for 4 tubs of vanilla frosting and color 1 tub of frosting red, 1 tub of frosting blue and leave the other 2 just white for a little more frosting, depending on how much frosting you want to put on your cupcakes.
Once the cupcakes are cooled, add some red color to one of the vanilla tubs of frosting and add some blue to another tub of frosting. Leave the other tub (s) as is, vanilla white.
Once you've attained your colors, lay out a piece of plastic wrap and start spooning the colors in a row, starting with red, then white, then blue. At this time you can even add more gel to the top of the red and blue colors by using a toothpick, if you want some even deeper color swirls before the next step.
Form the frosting into a log by rolling up the plastic wrap and twisting the ends. Then cut off one end of the plastic wrap.
Fit a piping bag with a large star piping tip, then place the frosting log into the bag with the cut off end facing towards the star tip.
Start piping the red, white and swirl frosting onto your cupcakes.
Add sprinkles to the tops and enjoy the celebration!