Preheat your oven to 300°F (150°C) and line a baking sheet with parchment paper.
In a medium bowl, sift together the powdered sugar and almond flour. Set aside.
In a separate bowl, beat the egg whites with an electric mixer until foamy. Gradually add the granulated sugar and continue to beat until stiff, glossy peaks form.
Gently fold the dry ingredients into the egg-white mixture until just combined.
Divide the batter into three separate bowls. Add red food colouring to one bowl and blue food colouring to another bowl. Leave the third bowl plain.
Transfer each coloured batter to a piping bag fitted with a round tip.
Pipe small circles onto the prepared baking sheet, leaving space between each one.
Tap the baking sheet on the counter to release any air bubbles, then let the macarons sit at room temperature for 30 minutes to form a skin.
Bake the macarons for 15-18 minutes, until set. Allow them to cool completely on the baking sheet.
While the macarons are cooling, make the buttercream filling. In a bowl, beat the butter until creamy. Gradually add the powdered sugar, milk, and vanilla extract, and beat until smooth.
Transfer the buttercream to a piping bag and pipe a small amount onto the flat side of half of the macarons.
Sandwich the filled macarons with the remaining plain macarons.