I did paint some loco head and tail lamps; they are a mixture of Roundhouse and GRS cast white metal lamps.
The lamp bodies were painted with matt acrylic white, the headlamp lenses were polished with a fine brass wire brush attachment fitted in a Rotacraft drill, and then given a thick coat of gloss varnish. The tail lamp lenses were painted with BR Maroon enamel paint - just happened to be the maroon I had in stock - and then coated with the gloss varnish. Initially I was going to paint the tail lamp lenses in a brighter signal red; you may have noticed however, that the lenses of the full size lamps are actually quite dark in colour and only appear to be bright red when they are lit. These are my daytime lamps - I have some working lamps for night use - so I opted for the darker shade of red.