The lens colour determines how much light will pass through a lens.
To keep vision optimal you need less light in sunny conditions and more in cloudy weather. Yellow: These lenses actually work as light enhancers. Good for dusk and low light as well as for overcast days and evenings. Brown: A superb all-round constant density lens. It provides the best combination of true colour transmission and contrast. The optimal performance is achieved in medium bright to bright conditions.