Absolutely nothing if my recent experiment is anything to go by. Note to self, food colouring is not easy to use!!
I couldn't find a green food colour anywhere so I improvised, blue and yellow make green right? Right they do but not a happy shiny green more of a fungus green, it did not look delicious.
I also thought it might be "fun" to colour the inside of my cake blue...... Hmmmm. Not only did my hands look like they'd just murdered a Smutf but I managed to stay on my fungus theme it was bluemould coloured cake with fungus coloured icing :-( I'm starting to worry now.
Ok - cake update, it actually didn't turn out too bad - for a first attempt, and it tasted lovely. Although I forgot to add the wheels and didn't notice until after it was cut (they were Oreos).