Affinity Designer 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 Designer provides the Colour panel's HSL colour wheel by default or optional sets of sliders. Furthermore, 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.