In the bowl of a stand mixer fitted with the paddle attachment, beat butter, lemon zest, and salt at medium speed until creamy, about 1 minute, stopping to scrape sides of bowl. With mixer on low speed, gradually add confectioners’ sugar alternately with cream, beginning and ending with confectioners’ sugar, beating until combined after each addition. Increase mixer speed to medium, and beat until fluffy, about 3 minutes.

Spoon 3 tablespoons (38 grams) buttercream into a small bowl; stir in red and black food coloring until desired red shade is reached. Stir green, yellow, and black food coloring into remaining buttercream until desired green color is reached.