Affinity Publisher can use different colour models to represent different ways of describing colour. These models (RGB, CMYK, Lab, and Grey) can be selected as you create your new document.
Colour definitions are described comprehensively on the internet, so in this section we'll just focus on how to set a colour model, the model's colour space, and how a colour profile represents that colour space.
In terms of selecting colour as you design, Affinity Publisher provides the Colour panel's RGB sliders by default or CMYK, HSL and LAB sliders; an HSL colour wheel can also be used. The Swatches panel can be used for easy access to preset and custom 'saved' colours that you'd like to use again.
If you're happy to leave colour management until output is required, you can create new documents with the suggested default colour format RGB/8 with sRGB IEC61966-2.1 colour profile. However, for professional printing it's recommended to ensure a CMYK colour profile is adopted on document creation.