How do I colour ready-to-roll icing

Although Aaronut is entirely right in stating that you would be better off using alternate colourings that do not damage the texture of your icing, you can use normal food colourings, I do quite often.

When I use them on fondant or marzipan, the main issue is the capacity of the icing to absorb the colouring. They have limited ability to tolerate the liquid, so if you need a deep colour, expect a bit of a fight.

The best method I have come up with, is to treat it like pulling candy. Put the required amount of icing in a bowl and make a well and enough material to fold over that well. Put in a small amount of colouring and fold over. I tend to start off by squidging it about in folding motions to get the liquid in to the material, it'll be quite tacky at first. Then I roll it between my hands in to a sausage, fold in half lengthways, roll, fold, roll, fold...

It'll marble up to start with, given the folds, but eventually become one solid colour and will become less tacky with each fold, in a similar way that kneading bread makes it progressively more coherent.

Once settled, roll towards a ball/block rather than a sausage, then you can roll it out ready for use. If you are a very minute amount over the moisture level that you can roll it, try to incorporate a small amount of icing sugar (confectioner's) whilst folding to dry it out, like adding flour to bread.

It works, but is messy, take all rings off and expect to need to wash your hands a lot.

Edit

Found a video that roughly shows how I would do it, except the demonstrator has a different way of applying the colouring to the fondant initially and is using a much larger quantity than I tend to, so does not hand roll in the air as much as I would or as fast.

http://www.ehow.com/video_2333485_coloring-cake-fondant.html

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.

I haven't worked with rolled icing / fondant much, but I did find a method that based on surface contact might be slightly faster than Orbling's repeated folding technique. (note, I was using gel colors, but I think it might work with liquid colors, too):

(okay ... I do two types of 'rolling', so it's kinda confusing ... 'roll out' is using a rolling pin to flatten; 'roll up' is turning the flat sheet into a log, like a jelly roll)

1. knead for a while 'til it gets softened up so you can roll it out.
2. roll out into a vaguely square or rectangular shape.
3. start rolling up one side
4. spread your color along the roll, but not too close to the ends.
5. continue rolling up (if there's too much color, it'll squish forward as you roll it)
6. repeat steps 2/3/5 'til the color's uniform, or just kneed as if it's dough; add in another 2/3/4/5 if the color isn't deep enough.

(basically, with Orbling's method, you're using roughtly half of the surface area ... rolling gets more used with each pass).

If the liquid's really a problem, you could also try taking a sharp knife, holding it at an angle, and slashing the surface (you want to cut open little flaps in it, but not cut through to the board below) ... this would further increase the surface areas to allow it to take more liquid per addition.

Instead of adding your colour to the total amount of fondant, try adding it first to a fist-sized ball of fondant till evenly mixed and then add that ball to the rest of the fondant. If the colour is not deep enough, repeat the procedure.

I like to knead my fondant like the bakers do with bread. Dust the table with a small amount of either icing sugar or cornflour and use a push and fold motion using the ball of the hand, keeping the fingers up and out of the way. Using fingers and flattening it encourages air bubbles. Keep the fondant compact and rounded as much as possible.

When I roll out the fondant, I use a dusting of cornflour on the table - works well and so easy to clean up later!