CMYK or RGB?
CMYK is also called 4-color Process. Colors in CMYK images are composed of varying amounts of Cyan, Magenta, Yellow and Black. This is the way a printing press prints color, so it is vital that your image be in this color mode. For 1-color Process it needs to be grayscale.
Comparisons between RGB displays and CMYK prints can be difficult, since the color reproduction technologies and properties are so different. A computer monitor mixes shades of red, green, and blue to create color pictures. A CMYK printer instead uses light-absorbing cyan, magenta and yellow inks, whose colors are mixed using dithering, halftoning, or some other optical technique. Similar to monitors, the inks used in printing produce a colour gamut that is “only a subset of the visible spectrum” although both colour modes have their own specific ranges. As a result of this items which are displayed on a computer monitor may not completely match the look of items which are printed if opposite colour modes are being combined in both mediums. When designing items to be printed, designers view the colours which they are choosing on an RGB colour mode (their computer screen), and it is often difficult to visualise the way in which the colour will turn out post printing because of this.