I just did it in a WWI NCO pilot figure for the LI Show in this way: After I sculpted the leather flying helmet, then the goggles on top of it (he had them up and on the front of the helmet), I built up a layer of gloss coat in side the goggle frame about 10 times.
This was done night after night, while working on other figures...but each night, a new thin layer. I used Modge Podge Gloss...goes on White, dries crystal clear.
I eventually wound up with a clear, glass like, goggle lens with depth and it "naturally" shaded the brown leather of the flying helmet beneath it a darker shade.
I'm doing some more pilots now, and will use this method...if your not in a rush, it's good.