Use liquid or powder food colouring to tint your icing a beautiful mint green colour. Liquid -- or gel -- food colouring offers a bolder colour output, while powders are more subtle. Whichever type of colouring you choose to use, it's best to experiment with using the colours in a small batch first. Colouring output varies by manufacturer and type.
Place a tablespoon of buttercream icing in a small bowl to experiment with colours in a small batch. Add one part leaf green, one part yellow and two parts blue. Stir with a spatula.
Set the icing aside for an hour to allow the colour to set, as colours darken with time. Make adjustments in your colour recipe and, if necessary, perform another trial run.
Create a lighter shade of mint by adding more of the plain white buttercream to the mixture. Fix a shade that's too yellow by adding more blue, by the drop, until the desired colour is reached.