I reckon just buy white icing (maybe royal icing?) from the supermarket and buy fondant while you are there, too. You can colour the fondant by kneading the colouring in. Just add the colouring gradually, until you got the shade you are happy with. Then roll out, cut into circles and place on iced cake. Just make sure you work fast with the fondant, otherwise it dries out and cracks. Definitely ice the cake first before working with the fondant. Oh, and try using proper colouring paste (wilton or similar) to colour the fondant. The food colour from the supermarket is too liquidy and messes with the texture of the fondant. (In saying that I have successfully used supermarket food colouring when I was desperate; I just mixed the colour up with icing sugar first, to make a paste and then added it to the fondant.)

Maybe practice the deco on a trial cake first, before you commit?

Good luck hun!!