Postage Stamps//Colour experimentation

Now i have the design of my stamps i want to experiment a little with the colour of these to get them right. I know that the concept of the brief is about being ecological which would refer to green, but i don't really want to use that as it doesn't link in with the cars that much. Road signs which are about information//telling you things to do are red and blue, so i think using these two colours are the best ones to use.