hmm - I don't think dimming a tank's light is very popular, most of us are out to increase our light output!
I'd also think putting just about anything on the bulb itself will causing overheating, and will shorten the life of the bulb.
how about using a piece of tinted glass between the light and the tank.
or some regular glass, and painting it white, or using some of that cheap car-window tint.