How do I know when to use which one?

The easiest explanation is CMYK for Print & RGB for Screens.

CMYK is still used by most professional printing companies so if you’re looking to get something printed professionally, you need to be aware of this. In short, if you’re going to be printing something, such as a business card, stationary, or a newsletter, use CMYK. CMYK does not include a white colour because it is assumed that it will be printed on a white paper and depending on the percentage of each colour that is used, the white from the paper will be used to fill the space, therefore making the shades appear lighter. Continue reading Choosing Colour for Print CMYK or RGB