I mixed 3.5kg of white fondant icing with pink sugarflair colouring today. You don't need the microwave. Just break it into manageable chunks, knead each one until soft, then mix together, add a tiny bit of colouring, and knead through until uniform in colour.
I'm talking sugarpaste. I haven't used gel colours before, might try to get some tomorrow then. I haven't been able to find the right shade of blue in a ready-coloured icing and I also need quite a large amount so didn't fancy kneading <lazy>