Using a small paring knife, cut a 1-inch circle into the top center of each cupcake and use a finger to press the middle of the cut-out to pack the cake down, making a well. Be careful not to press through the bottom. Fill each well flush to the top of the cupcake with galaxy sprinkles, about 1 tablespoon per cupcake.
Divide the buttercream evenly between 3 small bowls. Add the teal, pink, and purple food coloring to one bowl each and mix well. Transfer each color frosting to a piping bag fitted with a large round tip.
On a large sheet of plastic wrap, pipe a row of each color frosting. Gently roll up the plastic wrap around the frosting, then carefully transfer the roll to a large piping bag fitted with a medium star tip.
Pipe the swirled frosting onto the cupcakes, then sprinkle with galaxy sprinkles.