1Frost cooled brownies with chocolate frosting.
2Spoon vanilla frosting into 3 small microwavable bowls. Tint 1 bowl of frosting neon blue, 1 neon pink, and 1 neon yellow. Microwave each bowl of frosting until frosting melts.
3Transfer melted frosting into 3 small resealable plastic bags. Cut off small bottom corner of each bag; drizzle frosting over top of brownies. Let frosting cool and set before cutting brownies into 4 rows by 2 rows.