Over time and trooping, mine has become "weathered". Very cool idea. However, best to get your trooper costume in order, get approved, then worry about weathering. I did paint my chin Vocoder flat black, and it looks cool to me.
Also, from the Shadow CRL: "Lenses must be sufficiently dark enough to obscure the costumer's eyes. They may be flat or bubbled. Acceptable colors are: black, smoke, dark green, or silver."
You can read more here: http://www.501st.com/databank/Costuming:TX_shadow_stormtrooper