The act of colour picking lets you sample colours within Affinity Photo 2, then use them throughout your design.
Colour sampling appears in two slightly different forms within Affinity Photo 2. It exists as a panel 'component' as well as a standalone tool.
Both versions function in slightly different ways:
Regardless of which you use, a sampled colour is stored and applied from the swatch next to the colour picker's icon in the Colour panel.