Anyone with normal color vision agrees that blood is roughly the same color as strawberries, cardinals and the planet Mars. That is, they're all red. But could it be that what you call "red" is someone else's "blue"? Could people's color wheels be rotated with respect to one another's?

"That is the question we have all asked since grade school," said Jay Neitz, a color vision scientist at the University of Washington. In the past, most scientists would have answered that people with normal vision probably do all see the same colors. The thinking went that our brains have a default way of processing the light that hits cells in our eyes, and our perceptions of the light's color are tied to universal emotional responses. But recently, the answer has changed.

"I would say recent experiments lead us down a road to the idea that we don't all see the same colors," Neitz said.

Another color vision scientist, Joseph Carroll of the Medical College of Wisconsin, took it one step further: "I think we can say for certain that people don't see the same colors," he told Life's Little Mysteries.

One person's red might be another person's blue and vice versa, the scientists said. You might really see blood as the color someone else calls blue, and the sky as someone else's red. But our individual perceptions don't affect the way the color of blood, or that of the sky, make us feel.

Some sort of perception

An experiment with monkeys suggests color perception emerges in our brains in response to our experiences of the outside world, but that this process ensues according to no predetermined pattern. Like color-blind people and most mammals, male squirrel monkeys have only two types of color-sensitive cone cells in their eyes: green-sensitive cones and blue-sensitive cones. Lacking the additional information that would be picked up by a third, red-sensitive cone, the monkeys can only perceive the wavelengths of light we call "blue" and "yellow;" to them, "red" and "green" wavelengths appear neutral, and the monkeys cannot find red or green dots amid a gray background. [How Dogs See the World]

In work published in the journal Nature in 2009, Neitz and several colleagues injected a virus into the monkeys' eyes that randomly infected some of their green-sensitive cone cells. The virus inserted a gene into the DNA of the green cones it infected that converted them into red cones. This conferred the monkeys with blue, green and red cones. Although their brains were not wired for responding to signals from red cones, the monkeys soon made sense of the new information, and were able to find green and red dots in a gray image.

The scientists have since been investigating whether the same gene therapy technique could be used to cure red-green color blindness in humans, which affects 1 percent of American men. The work also suggests humans could one day be given a fourth kind of cone cell, such as the UV-sensitive cone found in some birds, potentially allowing us to see more colors.

But the monkey experiment had another profound implication: Even though neurons in the monkeys' brains were wired to receive signals from green cones, the neurons spontaneously adapted to receiving signals from red cones instead, somehow enabling the monkeys to perceive new colors. Neitz said, "The question is, what did the monkeys think the new colors were?"

The result shows there are no predetermined perceptions ascribed to each wavelength, said Carroll, who was not involved in the research. "The ability to discriminate certain wavelengths arose out of the blue, so to speak — with the simple introduction of a new gene. Thus, the [brain] circuitry there simply takes in whatever information it has and then confers some sort of perception."

When we're born, our brains most likely do the same thing, the scientists said. Our neurons aren't configured to respond to color in a default way; instead, we each develop a unique perception of color. "Color is a private sensation," Carroll said. [How Colors Got Their Symbolic Meanings]

Emotional colors

Other research shows differences in the way we each perceive color don't change the universal emotional responses we have to them. Regardless of what you actually see when you look at a clear sky, its shorter wavelengths (which we call "blue") tend to make us calm, whereas longer wavelengths (yellow, orange and red) make us more alert. These responses — which are present not just in humans, but in many creatures, from fish to single-celled organisms, which "prefer" to photosynthesize when the ambient light is yellow — are thought to have evolved as a way of establishing the day and night cycle of living things.

Because of how the atmosphere scatters sunlight throughout the day, blue light dominates at night and around midday when living things lie low, to avoid darkness or harsh UV light. Meanwhile, yellow light dominates around sunrise and sunset, when life on Earth tends to be most active.

In a study detailed in the May issue of the journal Animal Behavior, Neitz and his colleagues found that changing the color (or wavelength) of ambient light has a much bigger impact on the day-night cycle of fish than changing the intensity of that light, suggesting that the dominance of blue light at night really is why living things feel more tired at that time (rather than the fact that it's dark), and the dominance of yellow light in the morning is why we wake up then, rather than the fact that it's lighter. [Busting the 8-Hour-Sleep Myth: Why You Should Wake Up in the Night]

But these evolved responses to color have nothing to do with cone cells, or our perceptions. In 1998, scientists discovered a totally separate set of color-sensitive receptors in the human eye; these receptors, called melanopsin, independently gauge the amount of blue or yellow incoming light, and route this information to parts of the brain involved in emotions and the regulation of the circadian rhythm. Melanopsin probably evolved in life on Earth about a billion years prior to cone cells, and the ancient color-detectors send signals along an independent pathway in the brain.

"The reason we feel happy when we see red, orange and yellow light is because we're stimulating this ancient blue-yellow visual system," Neitz said. "But our conscious perception of blue and yellow comes from a completely different circuitry — the cone cells. So the fact that we have similar emotional reactions to different lights doesn't mean our perceptions of the color of the light are the same."

People with damage to parts of the brain involved in the perception of colors may not be able to perceive blue, red or yellow, but they would still be expected to have the same emotional reaction to the light as everyone else, Neitz said. Similarly, even if you perceive the sky as the color someone else would call "red," your blue sky still makes you feel calm.

This story was provided by Life's Little Mysteries, a sister site to LiveScience. Follow Natalie Wolchover on Twitter @nattyover. Follow Life's Little Mysteries on Twitter @llmysteries. We're also on Facebook & Google+.