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Although researchers previously believed that dreams were only black and white, they now know that most dreams are colorful. But how do the dreams of colorblind people look?

That depends on when they became colorblind. Because humans dream about what they know, people who become colorblind after birth can "see" colors in their dreams, according to "Colour Blindness: Causes and Effects" (Dalton Publishing, 2002).

However, people who are born completely colorblind and can only see their surroundings in black, white and shades of gray, do not know what colors look like, and therefore, their minds have no memories from which to fabricate colorful dreams.

Complete colorblindness, a visual condition also known as total color vision deficiency (CVD) or achromatopsia, is extremely rare and affects only an estimated one in 30,000 people worldwide, according to the National Library of Medicine (NLM).

Red-green color vision defects (in which a person has trouble distinguishing between reds and greens) are much more common and make up about 99 percent of color vision defieiencies, according to "Colour Blindness: Causes and Effects." Among populations with Northern European ancestry, red-green color vision defects occur in about 8 percent of males and 0.5 percent of females, according to the NLM.

A person with a red-green color vision defect will dream in the same color set that they see when awake. For example, in his or her dreams, the American flag will have moss-green stripes instead of scarlet.

In the 1950s, dream researchers commonly believed that people only dreamt in black and white, even though both previous and later dream research studies established that dreams have color, according to a 2002 study by Eric Schwitzgebel, a philosophy professor at the University of California at Riverside.

The growing popularity of black and white films in the 1950s as well as increased affordability of black and white television sets may have played a role in the phenomenon of people with full sight having colorblind dreams.

"The first half of the twentieth century saw the rise of black and white film media, and it is likely that the emergence of the view that dreams are black and white was connected to this change in film technology," Schwitzgebel stated in his study, which appeared in the journal Studies in History and Philosophy of Science.

In the 1960s, as media began to shift to full color, reports of black and white dreams became increasingly rare, showing that the things people observe during the daytime leach into their dreamscape.

"Only very, very rarely does someone report a dream that is black and white like an old movie," said Deirdre Barrett, a psychologist at Harvard Medical School who is an expert on dreaming.. "If they're not color blind, this may be due to exposure to old films."

Barrett notes that if you can't remember the colors in last night's dreams, that doesn't necessarily mean that you dreamt in black and white. Some people may focus on the colors within a dream, while others don't notice what color things were. This selective perception of sorts is similar to how different people perceive the real world, Barrett said.

"If I asked you to describe something that happened two days ago, you might include no color in the incident - or you might," Barrett told Life's Little Mysteries. "If I asked you what color someone was wearing, you might be able to tell me or notcolor just isn't always a salient part of events. The events may be more about the interpersonal aspect, such as navigating or trying to get somewhere, while at other times, color is significant to whatever we're doing and noticing."

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