The Forgotten Planet by Murray Leinster

It was near to sunset. Burl had never seen the sun, so it did not occur to him to think of the coming of night as the setting of anything. To him it was the letting down of darkness from the sky.

The process was invariable. Overhead there was always a thick and unbroken bank of vapor which seemed featureless until sunset. Then, toward the west, the brightness overhead turned orange and then pink, while to the east it simply faded to a deeper gray. As nightfall progressed, the red colorings grew deeper, moving toward mid-sky. Ultimately, scattered blotches of darkness began to spot that reddening sky as it grew darker in tone, going down toward that impossible redness which is indistinguishable from black. It was slowly achieving that redness.