Mix a drop of food colouring in a bowl with a little bit of water.
I used one drop of blue food colouring and 5 drops of water, and that coloured one big marshmallow.
Dip the cotton swab in the mixture and then use it to "paint" the marshmallows.
Let the marshmallows dry and then they're ready to use!
Note: If you're cutting the marshmallows in half and then sticking them on something or something like that then it's easier to do that first and then just "paint" the marshmallow once it's on the cupcake or whatever it is.