Just imagine a lightspectrum

http://hidden-technology.org/images/pics%20for%20site/lightspectrum.jpg


The human eye has receptors for 3 colorranges
which have their medium approximately at
red green and blue.

http://upload.wikimedia.org/wikipedia/commons/c/c2/Cone-response.png

For a human to see yellow, both the green and
red lightemitters (in the computerscreen) need to be on
(in about the same strenght).
Without blue beeing active.

If blue is active, the yellow will get brighter, up to
reaching a white-yellow.

There are other basecolors used in Printing media for example,
(magenta, cyan and yellow)
All have the purpose to simulate a full spectrum
by adjusting their values accordingly.

A true yellow can only be archived by emitting light in
the specific frequency (around 590nanomater). But the human eye approximates
yellow anyhow using its 3 color receptors.