When I first wanted to try doing this, I asked Mr Google and he told me to put the food colouring down one side on the inside of my piping bag. The more I thought about this, the more I was worried that it would be easy to use far too much colouring and also that it could easily produce a very uneven look to the colour. So I came up with the following way of doing it.
First I put half of my uncoloured buttercream onto a piece of clingfilm:
Then I coloured the other half and put it on top of the uncoloured pile:
I then folded the clingfilm over the top and squished the two piles together:
I tied one end of the clingfilm into a knot to stop the buttercream squeesing out, and I cut the other end up close next to the buttercream. I then put the 'package' into a piping bag fitted with a 1M tube and started piping. Simples!!
The effect is quite random (the first swirl had hardly any pink in it) but that is the beauty of the two tone swirl - each cupcake is individual.
There's no reason why you couldn't adapt this technique to do a third colour, although I fear that four or more colours might look a little 'busy.
