In an electric mixer, combine the room temperature butter and beat until smooth.
Add in the sugar and the vanilla and continue to beat until smooth and fluffy.
Divide the buttercream into four bowls and dye each one a different color. Pink, blue, green, yellow and purple. Use about one to two drops for each color.
In a piping bag, with the fitted tip scoop a small amount of each color next to each other, or on top of. Repeat until the bag is full. You want a rainbow effect, so you can't scoop all the color in at once, it needs to be divided throughout the bag to come out with all the colors.
Pipe the frosting onto each marshmallow filled cupcake and add a Lucky Charm to the top!