Colour systems for print.

subtractive colour - screen based - RGB

additive colour - print based - CMYK

RGB image - Image - View - Gamut Reading.

Terminology:

CMYK - cyan magenta yellow key

RGB - red green blue

Grayscale - grey tonal black and white image

Duotone - use of two colours in images

Spot Colour

Mono - one colour

Gamut reading.

RGB image - image - view - Gamut reading.

to be able to see the image in CMYK mode.

The image below shows the range of colours in RGB then highlights the CMYK colours within the triangle.

When printing at a printers you can ask to see there colour swatches before you print as to adapt your design or colour palette to make sure the colours are exactly what you wanted.