It's brain conditioning-darkness makes the brain lose function, just like rarely used muscles will atrophy
"Dreaming might be a kind of screen saver in which it doesn't really matter what the content is as long as certain parts of the brain are active" - Steven Pinker
About 80% of all dreams are in color, there's a small percentage of people who claim to only dream in greyscale. In studies where dreamers have been awakened and asked to select colors from a chart that match those in their dreams, soft pastel colors were most frequently chosen.
Whiteman, Honor. "Emotional impact of nightmares 'rarely due to fear,' study shows." Medical News Today. MediLexicon International, 01 Feb. 2014. Web. 15 Mar. 2017.
"Changes in Sleep with Age." Changes in Sleep with Age | Healthy Sleep. N.p., 18 Dec. 2007. Web. 15 Mar. 2017.
Cardoso, Silvia Helena, Ph.D. "Do We Need To Dream?" Do We Need To Dream? State University of Campinas, 1997. Web. 15 Mar. 2017.
"How to Analyze Your Dreams (And Why It’s Important)." Psych Central. N.p., 17 July 2016. Web. 15 Mar. 2017.
Cherry, Kendra. "10 Facts You Should Know About Dreams." Verywell.com. VeryWell, 14 Oct. 2016. Web. 15 Mar. 2017.