Smoke or red tint, over headlights or inside the reflector, is a common custom modification for enthusiast cars. For example, the rear tail lights of an '08-11 Subaru WRX are a style ripoff from the previous Honda Odyssey, and they have a frosty chrome interior and red LED bulbs, giving them a huge ratio of pointless chrome to functional reflectors and making them stick out like a sore thumb. Some of the earlier ones also have a big chrome bar in between the two lights. To subdue the look, people tint all the parts that don't make brake light. I've seen it on rims too. I don't know how the product works chemically, but in the sense of how it works functionally, it's paint.

Products:
Duplicolor Metalcast
Krylon Stained Glass
Testors spray enamel has six transparent colors.