Sight is the primary sensory perception utilized by fly fishing game fish to detect an artificial fly in or on the water. Whether or not the fish sees your fly depends in large degree on the amount of visible light traveling through the water (the refractive index) being reflected from the fly; the lower the level of visible light, the less visible the fly.
One of the most valuable lessons a fly fisher can learn is that the visibility and color(s) of artificial flies appear much differently to fish in the water that they do to humans in the air. This is the result of two important factors; the vision of the species of fish you are pursuing and the attenuation of light in the water you are fishing.
Attenuation causes visible light to be greatly reduced within water as this region of the electromagnetic spectrum is mostly scattered and absorbed. As a result, the greater percentage of light which effectively travels through water is ultraviolet light and invisible to the human eye. The small percentage of visible light available in water is further decreased in lower light seasons; on overcast days, in cloudy water and/or choppy water conditions causing artificial flies to be even more difficult to be seen by fish. And of course, visible light is least available in fishing waters throughout oblique positioning of the sun; at dusk, through the night and into the dawn; during the precise periods of what can be the most intense feeding by the largest game fish of many species.