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.
If it is something that will only be seen digitally, use RGB. The Internet is set up to work exclusively with RGB colours and there is a simple explanation behind this. A digital monitor is made up of tiny units called pixels. These pixels are comprised of three light units, one for red, one for green, and one for blue. The RGB values are applied to these pixels, thereby setting the luminosity for each of the light units in each pixel.
RGB – Colour
RGB is the colour scheme that is associated with electronic displays, such as CRT, LCD monitors, digital cameras, and scanners. It is an additive type of colour mode that combines the primary colours, red, green, and blue, in various degrees to create a variety of different colours.
CMYK – Colour
CMYK colours. This is a four-colour mode that utilizes the colours Cyan, Magenta, Yellow and Black in various amounts to create all the necessary colours when printing images. It is a subtractive process, which means that each additional unique colour means more light is removed, or absorbed, to create colours.