Lenses are often tinted for a variety of reasons. Sometimes the color is just accent to the already stylish frame. Other times the tint may help a person who is sensitive to bright lights (photophobia). Migraine headaches or severe allergies can also contribute to light sensitivity.
Tints can be a “solid” color, where the color is the same throughout the lens, or tints can be a “gradient,” where the top of the lens is darker and the bottom of the lens is light or clear. This is useful for those who are sensitive to overhead fluorescent lights but when they are trying to read they want all the light they can get.