If you were using liquid colouring, just keep going! LOL.
Google a local cake deco or sugarcraft shop to find paste colouring. There are ones up and down the country - they will sell the paste or else eBay or else a mail order place like Almond Art or the one previously suggestion.
Sugarflair is the brand I prefer but Squires is the big brand in the sugarcraft world.
I ended up using a whole tub of paste once and just kept getting more lurid pink! Someone suggested adding a bit of black next time but I haven't tried it so don't blame me if it all goes horribly wrong!
OMG How much icing are you trying to colour and feed your kids?! LOL...
I've had the same tub of ruby for a few Christmases now... admittedly I only normally colour ponsiettas, holly berries, santa clauses, choir boys and various other Christmas creatures but I've done a few fire engine and kids cakes but have still only got through about a third of the tub.
If you do get the paste stuff, the pink won't ever go red... the deepest colour it will go is the shade it is sold as. I hope that makes sense...