There are approximately 10,000,000 (10 million) colors in the world.
Careful measurements of our visual system’s best performance have been made by psychophysicists (people who study human responses, like seeing color, to things in the world, like light). They have shown that we can see about 1000 levels of light-dark, 100 levels of red-green, and 100 levels of yellow-blue for a single viewing condition in a laboratory. This means that the total number of colors we can see is about 1000 x 100 x 100 = 10,000,000 (10 million). A computer displays about 16.8 million colors to create full-color pictures, really more than necessary for most situations. However, the answer is not quite so simple. What color looks like is greatly affected by the viewing conditions.
These conditions include the color of the lighting, the amount of lighting, and other colors in the scene. Colors also appear in different modes when they appear on different objects such as surfaces, light sources, or within volumes. Different people also have slight differences in the way they see color. Since we can see at least 10-million colors in a single viewing condition and the variety of viewing conditions and observers is endless, then the only truly correct answer is infinity. If we have 10-million colors, times 10-million lighting types, times 10-million lighting levels, times 10-million surrounding colors, times 6-billion people in the world, times 3 modes of viewing we get a really huge number. The result of that multiplication is 18 followed by 33 zeros (18,000,000,000,000,000,000,000,000,000,000,000), or 18 decillion. That might not quite be infinity, but is close enough since all those estimated numbers are probably on the low side. And there is no way to exactly measure each of them.
How Many Different Colors Are There
16 million is an estimate of how many different colors a 24 bit RGB computer monitor can simulate. This has no relevance to how many different colors the human eye can distinguish.
Just as there are infinitely many numbers between 380 and 740 – including all of the fractions, of course – so there are infinitely many wavelengths between 380 and 740 nanometers – the visible spectrum of light. 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 imply that the human eye can distinguish an infinite number of colors. If you consider the apparent color of two wavelengths of light, the color will appear to be the same once you get the two wavelengths close enough. How many different colors can the human eye distinguish? The Encyclopedia Britannica mentions that the human eye can distinguish wavelengths as close together as 1 nm apart in the blue-green and yellow areas of the spectrum, but only those 10 or more nanometers apart in the deep red and violet. If the entire visible spectrum, from 380 to 740 nanometers, could be distinguished at a resolution of only 1 nanometer, the total number of spectral colors would be (740-380), or 360; for a resolution of 10 nanometers, it would be one-tenth this, so the total number of spectral colors is somewhere between 36 and 360. However, this does not take into account the non-spectral colors: the different tints which may be obtained by adding white light (or paint), the different shades which may be obtained by reducing the relative amount of light (or adding black paint), and the other non-spectral colors obtained by mixing different spectral colors.
The actual estimate for how many different colors the human eye can distinguish varies between one and ten 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.
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.