Another way to do it is to use "RIT" fabric dye. It comes in a whole lot of colors. You dissolve the dye in boiling water, then dip the clear parts into it. The longer you keep the parts in the dye, the darker the "glass" will get. Kinda like dying Easter eggs!
Obviously you don't want to put the kit glass directly into boiling water... let the water cool a bit. The big upside to this technique is that you're actually dying the plastic, not painting it, so there's no problem with runs or dust getting imbedded into your tinted windshield, like when using sprays.