Both RGB and CMYK are modes for mixing color in graphic design. As a quick reference, the RGB color mode is best for digital work, while CMYK is used for print products.
When to use RGB?
RGB (Red, Green and Blue) is the color space for digital images. Use the RGB color mode if your design is supposed to be displayed on any kind of screen.
When to use CMYK?
CMYK (Cyan, Magenta, Yellow, Key/Black) is the color space for printed materials. Use CMYK for any project design that will be physically printed, not viewed on a screen. If you need to recreate your design with ink or paint, the CMYK color mode will give you more accurate results.