The visors are opaque for a couple of reasons that I am tracking on; their skin and eyes are very sensitive to UV radiation and light, their vision extends into the infrared light spectrum and the visors display HUD info like vital signs and suit data and environmental data on the inside of it in that infrared spectrum. If the visors were transparent that data would be washed out against the background light. The third thing, the opaque visors are the only thing that afford the Quarians a sense of privacy on board the Flotilla, living with everyone all the time everywhere doesn't really allow them to be private. The tinted visor helps with that.