Me and my girl-friend were talking about this earlier today and I literally spent my entire day off looking up the answer. I will admit, this will be a hard answer to explain, especially when the answer was tough to find.
16 million is an estimate of how many different colors a 24 bit RGB computer monitor can simulate and while most of us may not care how mant colors we can see in a computer, this does has a little relevance to how many different colors the human eye can distinguish. “True” color is also called 24-bit color . Here, each color is 8 bits, for a total of 24 bits. Since each color has 256 shades, we can multiply 256 for red, times 256 for green, times 256 for blue and get millions of colors, (256 x 256 x 256 = 16,777,216). Millions of colors are pretty much what’s accepted for a monitor’s colors to look “true” to the human eye. But I digress.
So, we have a general idea of how many “colors” we can distinguish or at least see in a computer screen.
But what about “real colors”? Both in and out of a computer monitor? How many colors are there in our world?
I’ve found that, just as there are infinitely many numbers between 380 and 740 – including all of the fractions, or number of wavelengths between 380 and 740 nanometers, the visible spectrum of light by the way. That doesn’t even count the different tints and shades obtained by mixing in white, black, etc. So, there is an infinite number of colors, if you look at it that way.
However, that does not mean that the human eye can distinguish an infinite number of colors. Our eyes are not that powerful. Other organisms can see colors that we humans are blind to. For example, honeybees can see colors in ultraviolet light that is invisible to humans, though they see less of the red end of the spectrum. Birds can see not only ultraviolet but also red light; notice the red coloring of many flowers that are pollinated by birds. Unlike humans and bees, which each have systems containing three different color-sensing visual pigments, birds’ retinas contain four different pigments. This implies that the RGB monitors that can mimic any color for a human eye would need to project an additional color in order to mimic all of the colors a bird can see.
The actual estimate for how many different colors the human eye can distinguish varies between about 1 and 10 million, depending on the reference which you consult. However, the perception of color varies from one person to another, so there can be no single number that is true for everyone. The number of different colors that you, as an individual, can distinguish also varies dramatically according to the conditions; it drops to zero in low light conditions, in which only the rod cells of the retina can function, as the cone cells of the retina are required for color vision.
Something else to take into consideration is that colors aren’t the sort of thing that exists somewhere in the universe, they’re just perceptions we have in response to different physical phenomena. The phenomena most closely associated with different colors are the various frequencies of electromagnetic radiation, and there are indeed an infinite amount of different frequencies that this radiation can have. Of these, only a very small selection fall within the range that we call “visible light”, corresponding to the spectrum of colors we are familiar with. However, it is a continuous spectrum and so there are an infinite number of possible values of frequency of visible light.
Today scientist are interested also in larger than threshold color differences: the type of differences that would make you reject a certain touch-up paint because it’s not a close enough match to the color of the paint of your car. Even with such a metric there are close to a million discriminable colors on a computer monitor which only can reproduce a fraction of the colors we can see out in the real world.