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This week I made my first motorbike helmet cake, I made up the grey sugarpaste using white icing and adde black paste colour, kneaded it all and it was the perfect shade. I covered the cake and left it overnight to finish the next day. To my horror when i took the lid off the cardboard box it had turned a dark teal green! I had to have a quick dash to the supermarket for some silver spray.

Anyone know why this happened or how to stop it it happening the future??
Thanks

I would suspect that the black food colouring was not thoroughly mixed and you picked up too much of the yellow and blue pigment contained in it. Colour your icing the day before and allow to rest in order to see its true colour.

There is more discussion on the subject here http://cakesdecor.com/topics/1276-why-did-my-grey-fondant-turn-green