The grey filter was probably a polarisation filter to cut out sun-glare. (could be an ordinary darkening filter if needed to darken the sun for taking sunsights, but then it would be almost black like welding-glass or kaleuns would burn their eyes) Polarising works by allowing lightwaves to 'vibrate' one way but not at right angles to it. Doesn't make sense? Me neither. (Wiki:
http://en.wikipedia.org/wiki/Polarization) The first halve of the wiki-page is very mathmatical. But the lower part is much more 'down to earth'.
Since the game doesn't model that kind of light, it is of limited use to put a simple grey filter in. 'Realism' doesn't cut it if you ask me. I suspect it creates the opposite: less contrast. I personally would like to test different kinds of colors to see what works best for my eyes. So if you could provide some JSGME-ready files in different colors of the rainbow for that I'd be very happy.