You can use 1 teaspoon liquid pink food colouring or 1/2 teaspoon colouring paste.
I always like to use real ingredients not colourings or flavours so I replaced the pink colouring in the buttercream with freeze dried strawberry powder from ebay. The flavour was amaaaaaazing. There is no moisture in the powder so you can just add it in, no need to rebalance your recipe as you do when you add wet ingredients. - 15 Nov 2015