Find below the recipe for the buttercream to fill macarons, along with instructions on how to make the different colors. Also find instructions on how to airbrush the shells.
Airbrushing food coloring (I used periwinkle, yellow, green, pink, and purple)
Buttercream
1/2cupunsalted butter113 grams
1 to 1 1/2cupspowdered sugar125 to 187 grams
1/2tspvanilla extract
2tbspmilk if needed
food coloringI used periwinkle, egg yellow, green moss, pink, purple
Instructions
To airbrush the shells
Place a few drops of airbrush food coloring in the chamber of your airbrush machine. Turn the machine on and press the button to release food coloring over the shell while moving the machine side to side in a regular motion, in order to dispense an even amount of food coloring through the entire surface of the shell.
Don't linger over one spot, and don't keep the machine too close to the shell, because that will soak the shell, and make it disintegrate, or make the color uneven.
In between colors, make sure to clean the food coloring chamber with water and run it until the previous color is completely out before adding a different color.
Let the macarons dry before moving them around. It shouldn't take too long to dry.
Buttercream
Beat the butter with an electric mixer at medium high speed until smooth, for a few minutes.
Add 1 cup of powdered sugar and vanilla, mix on low to combine, and then raise the speed to medium high and beat for another minute or two. If the frosting is too dry add some of the milk, and if it's too soft, add more powdered sugar.
Divide the buttercream between 5 different bowls, or however many different colors you want to make.
To each bowl add a different food coloring. I used simply gel food coloring for the frosting. Mix each batch of frosting until incorporated and transfer it to a piping bag fitted with the tip of choice, I used a round tip to pipe the buttercream.
Make sure to keep the buttercream covered at all times because it can start to dry and form a crust.
Notes
Here is the recipe for making the macaron shells. Make sure the shells are white, or a very light color, or the airbrushing effect won't look so nice, or evident.