People describe light conditions as gloomy when the rods in their eyes take over from the cones and so their vision becomes shades of grey as they lose their colour vision. Low light of this sort is associated with depression. This association was made as far back as the 2nd century by the ancient Greek physician, Aretaeus of Cappadocia, who said, "Lethargics are to be laid in the light and exposed to the rays of the sun, for the disease is gloom."
The naturally weak daylight during winter at extreme latitudes causes winter depression. Also, weaker electrical activity is found in the retinas of depressed people, which gives them poor visual contrast so that they see the world in grey, depressing hues. A solarium or other source of bright light may be used as light therapy to treat this.
Photography of Akos Major "Highlands":