How do I colour ready-to-roll icing
There are special kinds of food colouring for icings, such as those sold by Wilton.  As a rule, you should use these, not traditional food colouring.

You can use ordinary food colouring but it will be hard to get the colours you want, and your icing will end up being watery/runny.  The icing colours (AKA gel icings) preserve the texture of your icing and, in general, are much more effective at actually getting the right colours.

You should be able to find these at any baking supply store.  There's no special technique, you just mix them.  They are completely fine to use on ready-to-roll icings as well as homemade.