(CNN) -- Violets are blue and roses are red, but maybe those colors are all in your head.
What does it mean for an object to be "red"? Is the way you perceive blueness the same as your neighbor? Your cousin? What about your dog?
Many scientists believe that humans have color vision that is generally consistent across populations and cultures, and that there are evolutionary reasons behind that constancy.
"Color vision is all about emotions and moods, and it has much deeper and richer connections to the rest of our perceptual worlds," said Mark Changizi, a cognitive scientist at Rensselaer Polytechnic Institute in Troy, New York.
Color vision in humans and animals
But some people really don't see the color red in the way that most do. About 8 percent of men have trouble differentiating between certain colors; less than 0.5 percent of women have this problem, according to the American Academy of Ophthalmology.
Color vision is based on photoreceptors in the eye called cones, of which there are about 6 million to 7 million in the human retina. Humans normally have three types of cones, corresponding to short, medium and long wavelengths of light. Purplish blues are at the short end, and reds are at the long end. They eye also has about 120 million rods, which detect light but not color.
According to some estimates, the human eye can distinguish about 1 million to 10 million different colors. A small minority of women actually have four kinds of cones in their eyes -- meaning they could theoretically see even more colors -- but only a genetic test can determine who has extra cones, and it's unclear exactly how differently they may see.
In most cases of colorblindness, the cone systems for either medium or long wavelengths do not work properly, resulting in reds, greens and perhaps yellows appearing very similar. But different people experience this to varying degrees. In rarer cases, people have trouble telling blue and yellow apart; the rarest of all make people see the world in grayscale.
Dogs and cats are generally colorblind, somewhat like humans who have trouble with reds and greens, but only see pale shades of color. On the other hand, they see better at night and have better peripheral vision. Insects see through photoreceptor units numbering in the hundreds or the thousands, almost like viewing the world as a mosaic. Some animals actually have better color vision than humans. Pigeons and goldfish, for example, can see ultraviolet light, which is invisible to people.
"We're great for mammals but pretty mediocre by broader standards," said David Hilbert, associate professor of philosophy at the University of Illinois at Chicago.
Biologists believe that animals' visual systems have evolved over millions of years and that the particular structures around today have persisted because they carried some survival benefit to the animals.
Why, then, do humans see in color?
One idea about the origin of the color vision of humans is that it helped early human ancestors spot red berries among green foliage in the wild.
More generally, color vision helps distinguish objects and their spatial structures from one another, said Stephen Palmer, professor of cognitive science at the University of California, Berkeley.
Changizi has a different idea. According to his research, the cones in our eyes are optimized to detect changes in hemoglobin as blood varies in oxygenation. In other words, what happens physiologically when you blush with embarrassment or turn pale with fear, we can better see with the full human color spectrum.
Across primates, those with furry faces tend to have more primitive visual systems -- with blue-yellow and grayscale perception -- and those with human-like visual systems, such as gorillas, chimpanzees, baboons and macaques, have bare faces. This is because, Changizi argues, the color vision allows bare-faced primates to detect changes in emotion and health according to very subtle changes in reds, yellows, greens and blues in the face.
To take advantage of that ability to detect health signals through skin color changes, Changizi recommends that hospitals adopt gowns that would approximate a variety of natural human skin tones. That would allow doctors to better perceive color changes in the skin than the currently available blue and white gowns, he said. More practically, perhaps, hospitals could have a catalog of hundreds of different skin tone tabs, to help the doctor's eye detect subtle tone changes.
So do most people have the same "red"?
For Changizi, this research on primates and color vision suggests that people's perception of colors must be consistent, given the way the eye's cones detect these specific subtle shifts in skin tone.
But other research suggests that seeing color can be something like tasting foods -- a pleasant flavor for one person may be too bitter or salty for another, just like a single shade of yellow could be seen as pretty or putrid by different people. This argument about color preferences is what Palmer studies in the Berkeley Color Project.
Generally, people naturally dislike tastes of foods that are molding or contain toxic substances. This aversion is evolutionarily beneficial because they are less likely to eat them and get sick.
"The idea is that the same thing is true for colors, that there are certain kinds of colors that are characteristic of stuff that's good for us as a species -- clear sky, clear water -- and stuff that's bad for us, like rotting foods, biological waste products," he said.
Blue may be a common favorite color because it signals such positive things as clear skies, he says. Yellow-green tends to be generally disliked, perhaps because it signals toxicity. A recent study from Palmer's lab that appeared in the Proceedings of the National Academy of Sciences found that participants' color preferences nearly perfectly corresponded with how much they liked objects of the same color.
Feelings about colors can change dramatically in response to a person's environment, and over the course of a lifetime, Palmer said. Institutional affiliation is also a factor -- a study from his lab found that Berkeley students tended to like their school colors, blue and gold, more than their rival school Stanford's colors, red and white.
"I don't think that we have a pure sensory experience of the color. I think it's overlaid with how much we like things," he said.
But most scientists still maintain that your "red" is probably the same as "red" for everyone else (who's not colorblind) in terms of what you see, the University of Illinois' Hilbert said.
"I think colors are out there in the world, and we see them more or less accurately, just like we do shapes and sizes," he said. "I don't think that makes them any less interesting or cool. In fact, I think it makes them more interesting."