One drop of green food coloring to one ounce of Future...mix. Dip those windows right in! Need it darker, dip again. Still not dark enough add another drop to the bottle.
Food Coloring is available in the baking supply section at your local grocery. A four pack of Yellow, Red, Green and Blue is a buck or so and lasts forever! Pick up a black one too, then you can use it to make smoked windows.