What are colors? If you go to any course about photography, you learn that all colors are made up of the colors red, green and blue (RGB). (If you mix red, green and blue light, you will get white light, or any other color according to how much of each light color you use.)
If you ask a physicist, he or she will tell you that the visible light is made up of all the colors in the rainbow. The water splits the white light and shows the rainbow with all the different colors it contains.
What is color, and what is it made of? Are all the colors in the rainbow really basic colors, and photographers have just experienced that you can reproduce all those colors with red, green and blue? What is the commonly accepted idea about colors?
 A: RGB is a property of our eyes, not of physics.  Our eyes only have 3 different types of color-sensing cells, so we can only perceive 3 regions of the rainbow, and mixtures of those regions.  Yellow light falls between red and green, for instance, and triggers both types of cells, but we don't have cells that respond only to yellow.
Mantis shrimp can perceive 12 different colors, as well as the polarization of light, which helps them see camouflaged or transparent prey.  The military does the same thing with hyperspectral cameras: http://www.bbc.co.uk/news/10175960
A: This has as much to do with biology as with physics.  The long answer on the biology is here.  The biology in summary: the human eye has only three different "color-sensitive" elements, and uses a complex combination of the amount of response it sees from each of these to assign a "color" to the image.  
Because there are only three sensitivities in the human eye, there are a variety of different techniques using only three basic (in some schemes called primary) colors to represent colors to humans.  
The actual frequency of the light emitted from the part of the rainbow we call green may have the same effect on the human eye as something we get by mixing our blue crayon with our yellow crayon on a piece of paper.  But it is easy to build a detector which will trivially differentiate between monochromatic green from one slice of a rainbow, and a mixture of the light reflected from blue and yellow crayon pigments.  
In principle, humans might have evolved a different eye with four or five different "color" detectors, in which case the schemes needed to make color images would probably need to have four or five basic colors, and the images we see from our current three color representations would seem to be washed out, missing something important.  But the eye didn't develop that way.
A: RGB is not 'basic'. It's just a convenient way of mixing colors. You could easily define your own set of three colors and get the whole spectrum(not sure--but you can obtain a large chunk of it.).
Color is just a property of the frequency of light. Change the frequency, you change the color.
Our eyes have rods and cones. The cones get stimulated by various frequencies of light, sending signals to the brain which are interpreted as color. That's it. That's what color is.
A: what we perceive  as light or colors is visible light region in electromagnetic spectrum http://en.wikipedia.org/wiki/Electromagnetic_spectrum . Human beings eyes can only perceive  light/colors in this specific region of electromagnetic spectrum(wavelength between 400-800 nm [nm=nano meter]), it is this electromagnetic radiation between this range we see as colors.for example light at the range of 400 nm wavelength is violet and that of 800 nm will be red. There are some animals which can see  ultraviolet or infrared( both cant be perceive  by human eyes) radiations because there eyes are sensitive to UV or IR part of electromagnetic spectrum
A: I would love to comment @Manishearth answer but I have no enough reputation to. So I have just to say that the electromagnetic visible spectrum is far more wide than the RGB space: each colour perception can be produced by an infinite set of spectral distributions (even attending only to the visible range, of course). Each one of that distributions have only to excite in the same amount the different kinds of cells.
It is usually very interesting to distinguish between how we detect light waves (its wave nature is not very important for that purpose) and sound waves.
