She says ‘periwinkle’, I say ‘blue’…

She says ‘periwinkle’, I say ‘blue’ (The invasion of the tetrachromats)
The way we perceive our world is a mere representation of reality (for an example, see this) that our brain makes through the usual five senses: sight, taste, smell, touch and hearing.  That said, our perception of the world by those senses is limited.  Nature is literally much more than what actually meets the eye (or the ear, or the nose, etc.).
Take sight, for example.  We humans are very visual beings.  Barring disease, developmental defects or accidents most of us have two eyes that point forward.  When we look at anything, the slight angle formed by each or our eyes is processed in our nervous system to give the impression of a tri-dimensional image (yes, we see in 3D; it was invented by evolution, not by movie makers!).  If you want to see in 2D, just cover one eye.  It is subtle change, but you’ll notice.
We are able to see because we detect photons, which are essentially particles of light.  Weird particles to be sure, because sometimes they behave like waves, but I will not get into that here.  For that I’d have to talk about quantum mechanics and I’d rather not get a migraine or give you one.  For our purposes, it suffices to say that we can talk about light in terms or either a particle or a wave.
Visible light is very small part of a much larger electromagnetic spectrum (see below), composed of types of light of various characteristics, usually expressed in terms of energy and wavelength.  What we call visible light is the arbitrary subset of the whole spectrum that we can actually see with our eyes, which corresponds to wavelengths roughly between 400-700 nm (a nanometer-nm, is a billionth of a meter = 0.000000001m).

Before getting into the basics of the mechanism of vision, it is important to realize that humans are able to detect to some extent, certain types of light other than visible light.  For example, if you place your hand near a hot stove (without touching it!) you will feel the heat.  That heat is a form of light indeed, at a wavelength that we cannot see but can perceive through our sense of touch.  We call it infrared because it is located “beyond” the red in the spectrum, as shown above.  At the other end, we can have ultraviolet light, which again, we cannot see, but can nonetheless feel as sunburn.  Other examples of light that we cannot detect without specialized equipment are microwaves (like the oven), X-rays, Gamma rays and radio waves.
We can call all these different wavelengths colors, even if we can’t see them.  Other types of organisms can actually see beyond what we can see, like honeybees and some butterflies for example, which can see in the ultraviolet.  Also, some snakes have specialized organs that can detect infrared radiation and so on.
In visible light, the multiple ways in which different wavelengths can be mixed together generates the roughly 1 million colors that most humans can detect.
Wait, a million colors?  How?
Humans detect light by a series of specialized cells called photoreceptors.  They come in two main types.  Rods work best in dimmer light, as they are specialize to detect light intensity.  There is only one type of rod, which has only a specific type of photoreceptive (light harvesting) pigment.  Therefore, they are not good at detecting colors.  When you look for something in a dark room, you may be able to detect shapes but no colors; in this case, you are using your rods.
The other type of photoreceptor cells are the cones.  Of these, we humans normally have three types, each of them most sensitive to light at a specific range of wavelengths.  The most numerous are the “red” cones (red is not their color but the light wavelength where they are most sensitive), followed by the “green” cones and lastly the “blue” cones.
Now here’s the thing.  There is a range of wavelengths that can be arbitrarily considered green.  For example, the figure below shows the approximate wavelength range for a green-like color:

Let’s suppose that for simplicity’s sake, we divide the “green” portion of the spectrum into 100 segments, each segment is a nm long that can be considered a form of “green”.
If we do the same for red and blue, the theoretical number of different colors that can be seen by combining the light that gets to the retina is:
100 x 100 x 100 = 1 million
The interesting thing is that some people (about 12% of the population, mainly women) have an additional type of cone, sensitive to either red or green.  We call those individuals tetrachromats (“tetra” means four, as in 4 cone types).  We “typicals” are trichromats (“tri” means three, as in three cone types).
That additional cone provides an increased capacity to perceive various wavelength combinations as distinct colors:
100 x 100 x 100 x 100 = 100 million!
It gets better; in theory, if we divide the wavelength segments by half, say half a nm instead of 1 nm, we could double the possible colors that can be perceived.
If you think that’s impressive, check out what the mantis shrimp can do.
Now, I know it is not that simple, but that may explain why it is easier for me choose a color to paint, say, a shelf.
I can either elect to paint it red or blue, for example (it’s a national election year, bear with me…).

And why, on the other hand my wife, whom I love very, very much, may have a slightly harder time choosing the color, because she may need to choose between:


If you want to know more:
http://www.nasa.gov/audience/forstudents/k-4/dictionary/Wavelength.html
http://discovermagazine.com/2012/jul-aug/06-humans-with-super-human-vision
http://www.post-gazette.com/stories/news/health/some-women-may-see-100-million-colors-thanks-to-their-genes-450179/
http://blog.lib.umn.edu/farre212/f11psy1001ds1415/2011/10/tetrachromats.html
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