Is there a food colouring that I can add to my cat's food to make him more orangy/red?

At your cats age I don't think it's a good idea to experiment with anything to change his color, as a matter of fact I don't think anyone regardless of the age of their pet should experiment with them. They are not science projects they are pets and just like with people as we age our hair turn gray. Leave it the way it is and that will insure a bad experiment doesn't cause him harm.