How do I colour ready-to-roll icing I'm making a cake for my daughter's birthday. I'm a bit rubbish with icing and decorating, so I plan to buy some ready-to-roll icing to cover the cake. The problem is that the cake is shaped liked a cartoon character, and I want to colour parts of it appropriately. My question is what is my best approach to colouring this icing? Is it possible for me to knead food colouring into the icing, or should I try something else? 

I've did previously try to shop-bought royal icing that I could microwave to a paste then colour, but the overall effect wasn't great (mainly due to my spreading technique!).
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.