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Has anyone used this before?

My other thought for the birthday cake is to make a rainbow cake which means I need really vibrant colours and I know that gels are better than liquids.

Our useless local supermarket doesn't sell them so a trip to Tescos could be in order but I wondered what they are like.

I have a tube of black, I got it in case I needed it but haven't tried it.
Amazon or a cake decorating supplier is a better bet; what you need is paste which colours best of all (I have red and green for Christmas stuff and you only need a couple of drops to get real red, use a cocktail stick).
This is where I got the coloured fondant:
http://www.cake-links.com/

They're really helpful and have lots of tips and how-to's on their site too

Tbh I think you're better with a cake that looks good from the outside - keep it simple - rather than one that only looks good once you cut into it