The lenses in the Airman are made from nylon, which is a lightweight material that can be made in many tint color options.
Choosing the right lens for you:
Grey Lenses are the most neutral option, as they control the brightness of light without affecting color perception, but do not provide the contrast of green or brown lenses.
Green Lenses provide a balance between the color neutrality of grey lenses and the high-contrast provided by brown lenses.
Gradient Lenses are a slightly lighter tint option that can be worn in lower light situations, but also can be used as a standard outdoor tinted lens.