here are a couple of links for you guys
http://www.legend.org/performance/tutorials/taillights.html
http://www.corrado-club.ca/mods/tailtints.html
Also, here is a secret how its really get done at the body shops
1. some automotive paint supply stores sell a "Grey Toner" then depends of how much clear coat you want to use and how dark you want, you put a few drops of grey toner in there. and then just spray it.
if you have air compressor you can get inexpensive gun at Harbor Freight Tool
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92126
for SoCal residents, if you go to that store you can buy it for $7
for those who does not have a compressor, check link below
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=1102
also a month ago I was over my friend body shop, he mixed me a paint (they called it "Shady Black". very amazing mix, color is black, and when the guy spraid it right on the headlight that I brought, and after that headlight was still clear, but if you look at the headlight, it looks like black color is on inside. plus that paint contains candy in it, and it makes look so amazing. I took a few pics, but you couldn't really see anything, only you can see that when you are in person.
pic. below is when I smoked Escalade lights with Grey Toner added to the clear coat