There are two ways to mix colours. If we mix light, we call it additive mixture; when we mix inks, dyes or pigments, it is subtractive mixture. There is really no difference in the way the colours behave in the two circumstances. Only the mechanics of the processes are different.  Additive colour mixture begins with the absence of light (black), and adds colours of light together to form new colours.

Subtractive  colour mixture is just like additive mixture in the way that colours of light interact, and stimulate our vision. Almost all practical usage of  colour involves subtractive mixture. Painting, printing, photography, and fabric dying are examples.

Subtractive mixture starts with the presence of all colours of light, usually as white light reflected from a white surface, such as a piece of paper. Then dyes, inks, or filters are used to subtract some of the reflected light.

The key to understanding subtractive mixture lies in understanding how colours of light are subtracted. If we put yellow paint or ink on a white piece of paper, it seems like we're adding colour to the paper. But the colour is already there; the white paper reflects all colours of light, approximately equally. The yellow ink, however, reflects only red and green light. It absorbs blue light, thereby subtracting it from the white light.

Any colour of ink, dye or paint subtracts its complementary  colour of light. Cyan ink on white paper absorbs red light, and allows green and blue to be reflected. Magenta ink subtracts green light, and allows red and blue to reflect. Yellow ink absorbs blue light, allowing red and green to reflect.

Cyan, magenta and yellow are often used as the subtractive primary colours. Combined in pairs, they produce the  colours red, green and blue. When all three primary colours are subtractively combined, they subtract all  colours of light, leaving black.

When two of the primary  colours are overlaid, they each subtract one  colour, allowing only the third  colour to be reflected. For example, if magenta and yellow ink are overprinted on white paper, the magenta ink absorbs green light. The yellow ink subtracts blue light. Neither of them absorbs red light, so the red light is reflected by the white paper, and a viewer sees the  colour red.

So, the  colours we experience in subtractive  colour mixture are created in the same way they're created with additive mixture. A combination of red and green light (where the red and green  colours each contain light from one-third of the spectrum) will always produce a yellow- coloured light (containing light from two-thirds of the spectrum). It doesn't matter whether we started with white light and subtracted one-third of the spectrum, or started with no light (black) and added two thirds of the spectrum.
Similarly, green and blue light always combine to produce cyan- coloured light, and red and blue light always combine to produce magenta- coloured light.

And complementary  colours work in similar ways for both additive and subtractive mixture. In additive mixture for example, yellow and blue light combine to complete the spectrum, producing white light. In subtractive mixture, though, yellow and blue produce black. Yellow ink subtracts one-third of the spectral light, blue ink subtracts the other two-thirds of the light, and nothing is left. No light is blackness.
 
 

Source: http://ourworld.compuserve.com/homepages/muser/submix.htm