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