In the bowl of a stand mixer or in a large bowl with a hand mixer, beat the butter until creamy.
Slowly blend in the confectioners' sugar, cup by cup until well-combined. Mix in milk.
Separate buttercream frosting into three separate bowls.
If you're using real raspberries instead of extract, place them in a saucepan over medium heat, stirring with a wooden spoon until they break down into a sauce. Continue to cook until sauce thickens. Remove from heat and pour through a fine mesh strainer. Press on seeds with wooden spoon to extract juice.
Mix 2 t fresh raspberry juice (or raspberry extract) into frosting in one bowl, mix orange extract into another, and mix fresh lime juice into the third.
Mix orange food coloring into orange extract frosting and green food coloring into lime extract frosting. If you used raspberry extract instead of fresh or frozen raspberries, mix red food coloring into raspberry frosting.
If frosting is too stiff, more milk can be added 1 tsp at a time.
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