Masks are generally available with either black or clear skirts, which have benefits and drawbacks for different conditions.
The black silicone skirt is a favorite among photographers because it blocks out unwanted light that causes reflections on the inside of the lens. Black silicone also shades your eyes and reduces glare, which can be helpful in tropical locales where the sun can often be so bright and the water so clear that sunlight bouncing off a shallow sand bottom can cause the warm water equivalent of snow blindness.
A clear silicone skirt, on the other hand, produces an open airy feel and, if the skirt is transparent, actually extends field of view. In more temperate waters where light is scarcer, clear silicone allows light to flow more freely into the mask. Divers who are prone to claustrophobia tend to feel less boxed in when wearing a clear silicone mask.
Each style has its benefits, and most masks are available in both. ScubaLab test divers bounce back and forth between the two, depending on diving conditions and activity. But the basic rule is, black skirts in bright water, clear masks in dark water.