How to decorate a thomas the tank cake without blue food colouring?

my two year old wants a thomas cake and i am totally against food additives especially the blue stuff.. see my problem? anyone out there with creative ideas on how to make it as thomas like as possible without having anything that's edible blue coloured? i have a candle already that is thomas shaped but that might look a bit lonely atop a stark white cake. i'm a totally beginner when it comes to decorating too. help please.