Discovered a great description on this web site poster printin. CMYK means Cyan, Magenta, Yellow and Black. These represent the 4 colours employed in conventional offset printing. These three colours and black roughly correspond to primary colours that are blended through the visible spectrum. This is the way printed colours are made. Adjusting your colours to CMYK will probably generate the most reliable portrayal of what the printing will look like. Have had the print out completed with them and it appears terrific.