what is "primary" refer to in visual

[Simon Thum]

Am 16.03.2010 09:42, schrieb Zhang, Xing Z:
> Hi experts:
>          From xlib manual, here are sentences for color:
> 
> GrayScale is treated the same way as PseudoColor except that the *primary* that drives the
> screen is undefined. Thus, the client should always store the same value for red, green, and
> blue in the colormaps.
> 
> TrueColor is treated the same way as DirectColor except that the colormap has predefined,
> read-only RGB values. These RGB values are server dependent but provide linear or
> near-linear ramps in each *primary*.
> 
>        I apologize for my bad English, I am confused by the word "primary". I wonder what does
> "primary" mean here? Thank you.

You may find this link helpful:
http://en.wikipedia.org/wiki/Primary_colour

Cheers,

Simon