This is my first post here at ARC, so I hope I am doing it correctly! I come from the dark side of car builders, and one of the tricks for tinting windscreens was to use clothing dyes. In the States, you can use RIT dye, and basically follow instructions like you were going to dye some clothes. The nice thing is that if it doesn't come out dark enough, you can re-dye it numerous times. I do not remember the exact article, but it was in either Scale Auto or Car Modeler along time ago.
David