the best way to tint lights is to get some legit black automotive paint (not duplicolor from autozone either)and reduce it out then clear it, it works awsome and you can use it on all lights and lenses. that is how my tail lights on my mustang are done and they are so glossy it will make your truck look scratched. I am going to be doing this for a small fee very shortly for any body that would want to send me there lights.
there's some spray tint called "VHT Nightshade" that most people use to tint their lights. I really like the look on taillights, but have never really been a fan of doing the front lights (on a Ranger at least)...
that vht stuff is garbage. it dosent let any light through at all. this way that I do it you could tint head lights as dark as you want and it dosent make a difference on the output of light. it makes the headlights look a little yellow, but if you have HID's or blue bulbs it dosent affect the color at all.