RGB and CMYK both describe the primary colors used in full-color reproduction systems.
Red, Blue and Green are used when reproducing full color using projected light on screen. Computer monitors, televisions, projectors, tablets, phones use RGB to simulate full color.
Cyan, Magenta, Yellow and Black (k) are used when reproducing full color using reflected light in print. CMYK is often called Process Color.
Both systems create the perception of a rainbow of colors by breaking their primary colors into dots of varying strength that are invisible to the eye.
Raster image color space
Digital photographs and raster image files are typically saved in RGB color space. When editing raster images in Photoshop, it is best to keep all photographs in RGB color space up until the moment you are ready to print.
Once design is done and you are ready to print, all raster images should then be converted to CMYK color space. Adobe Photoshop is the best application for this purpose. See our guide File Prepration Fact Sheet for additional information.
You should choose your color swatches by referring to a printed swatch book such as a Pantone Guide or CMYK Color Reference book.