We see color because our retina has three types of cones.  Roughly speaking, one is more sensitive in the BLUE, one in the GREEN and the third in the RED.  Our perception of color is based on how much each type of cone is excited by the light striking it.  If we get equal excitation of red and green cones with only a few blue cones excited, we perceive the color as yellow.  If mostly red cones are excited with fewer green and blue cones excited we perceive the color as a shade of red.  If all three are excited at roughly the same level, we perceive the light as white.  If equal amounts of red and blue light strike our eyes, we perceive the color as magenta.  An equal mixture of blue and green is cyan.  We perceive things as dark grey or black when there is little or no light present, i.e. black is the absence of light.

    As a result, almost all the colors we perceive can be produced by some combination of red, green and blue light.  That means that if you have three separate lights, one red, one green and one blue, you can shine them together to produce almost any of the colors we can perceive.  Thus light of 450nm (blue) , 550nm (green) and about 630nm (red) can be combined to mimic light of other wavelengths.  Equal combinations of red and green look yellow, so light of 550nm + light of 630nm will look to us like light of 590nm.  

    Because we can add these three colors, red (R) , green (G) and blue (B) in varying amounts to produce all the other colors we can perceive, we call them the primary additive colors.  A color TV or color monitor works by producing only three colors, red, green and blue and "mixing" them in various amounts to produce the colors seen on the screen.  (They actually make small dots of red, green and blue of differing intensities to make the different colors.  The dots are so small they appear as "one" dot to our eyes.  If you look at a TV with a magnifying glass, you can see the dots.  On computer monitors they are smaller yet, so they are hard to see even with a magnifying glass.  Try it.)  

Color Due to Selective Absorption (and Reflection)

    We perceive objects to be certain colors because they either produce certain colors, or reflect certain colors.  For instance, white paper appears white in sunlight because it reflects all colors and sunlight contains all colors.  Therefore, our eye receives all colors from reflections of sunlight from the white paper.  To our eyes, an equal mixture of all colors looks white.  If we shine only red light on white paper, it will appear red because it can only reflect what strikes it.  If we shine blue light on it it will appear blue.  

    What about colored paper.  Red paper appears red in white light because it reflects red light much better than blue or green light, absorbing most of the blue and green instead of reflecting them.  If it reflects a moderate amount of blue and green it will appear a lighter shade of red, e.g. pink.  If it reflects almost no blue and green it will appear a deep red.  Another way of thinking of it is that pink is a pure red with some white added in.  Orange paper reflects red best, but also reflects some green and very little blue.  If I illuminate red paper with green light, it will not reflect much light at all, absorbing most of the green and it will appear very dark green green or even black  (Note that any color will tend to look black if the light intensity is low enough.  The following sequence is yellow with lower and lower light levels.                                    .)

    Pigments and dyes typically work by selectively absorbing certain colors and transmitting or reflecting the ones they do not absorb.  In a sense they act by subtraction, removing certain colors from the light. 

Subtractive Coloration     

    Just as we can add red, green and blue lights together to produce other colors, we can also produce colors by taking white light and removing or subtracting colors from it, as with dyes and pigments.  For instance, to produce red light from white light we need to remove blues and greens. from the white light.  We say we filter out or subtract blue and green from the white light.  If we have a source of white light, e.g. the sun, we can produce red light by passing the light through such a filter that subtracts the blues and greens.  If I want to produce yellow from white light, I need to subtract the blues, so a yellow filter subtracts blue.  To produce cyan, I need to remove the red; and to produce magenta, I need to remove the green.  I can think of these as Yellow (Y), Cyan (C), and Magenta (M) filters.  Using a sequence of these filters, I can produce almost any color from White (W) light.  (I have to be able to adjust the amount of R, G or B they filter to get any color.)  For instance, If I want red, R, light from white light, I pass the light through a Y and an M filter in succession.  (The light passes through both filters.)  Since the Y subtracts B and the M subtracts G, all that is left is R.  I can think of this the following way, W = R + G + B, so when the magenta filter subtracts G I have W - G = R + G + B - G = R + B.  Now I run it through the yellow filter which subtracts B and I get R + B - B = R. 





Similarly, when I run white light through a Magenta and then a Cyan filter I get the following:  (R + G + B) - G - R = B.  




    Many printing inks work like this.  As a result one can do color printing with three inks, Y, C, and M.  

Color by Selective Scattering

There are many causes of color, but I'll just mention one more.  Small (very small) particles scatter some light, but they scatter more short wavelength light than long wavelength light.  This means that if you send white light through a region containing very small particles, like the air, more of the blues and greens are scattered than the reds.  After traveling many miles through the air, the light will acquire a reddish or orange tinge.  This is the case in sunsets.  However if you are looking at the sky in a direction that is "away from" the sun, you see the scattered light, which tends to be bluish.  That is why the sky looks blue and why sunsets tend to have a red tinge.