RGB
RGB stands for Red, Green and Blue – the three primary colours of light. Monitors, smartphones and cameras create colours by lighting these three at varying intensities. All three together produce white, none produces black. This is called additive colour mixing.
Photos and web graphics are almost always in RGB. For printing, however, RGB must be converted to CMYK, because printing uses ink, not light. Particularly bright tones are lost in the process – they look duller in print than on screen.
So the rule is: create print data directly in CMYK rather than supplying it in RGB.