Professional 4-color printing is done with MCYB (magenta, cyan, yellow,
black), while color television (or computer screens) is usually done with
RGB (red, green, blue).  I'm not sure precisely how to translate RGB to
MCYB, but I think an approximate color mix is:

Official
Color	Description	RGB Mix
Name

Magenta	Hot Violet 	Red+Blue, No Green ??
Cyan	Aqua		Green+Blue, No Red ??
Yellow	Yellow		Red+Green, No Blue ??

(Kate says Magenta is a kind of cool red, or better, azelia fucia.) 

If you specifically want to print a 24 bit color image on a monochrome
printer, one of the postscript utility files says that the color
information is converted to grayscale intensity values using the CIE
formula:

          intensity = .299 red + .587 green + .114 blue
