How do you paint a mudguard?

The simple answer is, "just like you would paint anything else." However, to my knowledge, mudguards are made of rubber, or a rubberlike, material, and most paints will not last very long before beginning to fall off. The only thing I can think of that MIGHT stick, fairly permanently, are the type of paints used on the plastic/rubber "bumpers" on later model automobiles. This type of paint is probably NOT available at your local auto parts store, so you would have to get it at a paint store that sold automotive paints. Also, someone at one of these stores, would probably know what, if anything, would work successfully. Good luck !