In a large bowl, beat the butter until smooth.
Slowly add in the powdered sugar and mix until all is combined.
Add in milk.
Separate the buttercream into 3 equal portions.
Wash and dry raspberries, then place them in a saucepan over medium heat. Stir and break down raspberries until they have broken down to a sauce. Remove from heat and pour raspberries through a fine-mesh strainer. Press out juice.
Mix about 2 teaspoons of the raspberry juice into the first portion of buttercream. Mix the orange extract and the orange food coloring into the second buttercream portion. Mix the lime juice and green food coloring into the third portion.
Put each portion of buttercream into its own piping bag, and pipe each color in a thick line, side-by-side on a piece of cling wrap. Roll up the buttercream in the cling wrap, cut the tip and slide the roll into another piping bag (prepared with a star tip).
Pipe buttercream swirls onto cooled sugar cookies.