normally rust will do that for you -- no charge, courtesy of Alfa Romeo's high quality italian metal!
Anyway what do you mean? colour, as in french style yellow lights?
There is a film you can get like decalfx or cleardefender, something like that?
Or yellow bulbs or yellow transparent paint for the bulbs themselves ....
Granville amber light maybe?- though this is designed for the bulbs themselves
or a film like lamin-x which is used in racing and protects from stone chips etc.
a lot of people use Krylon stained glass paint: Krylon: Products: Stained Glass Color