Basically, it's the main 4 colors used in color printing Cyan, Magenta, Yellow, and Black (CMYK) and they add the Green and Orange (LC and LM) to 4 color process to make it 6-color hexachrome printing. This offers more rich vivid color beyond that which the usual CMYK color model can attain. For more information, see Wikipedia's article on
Hexachrome Printing.
Submitted by Brian Behm of
Atonal Art.