Brown lenses are solid all around performers and ideal if you own just one pair of sunglasses. Brown works well in mid to bright conditions and highlight things against green backgrounds. Brown lenses naturally filter out high frequency light waves and as a result has an image sharpening effect. Because they perform well in both bright and dull conditions they are ideal if you live in an area that has frequent light condition changes such as overcast mornings changing to sunny afternoons.
Also known in the industry as copper and dark amber lenses.