The act of colour picking lets you sample colours within or outside Affinity Publisher, then use them throughout your design.
Colour sampling appears in two slightly different forms within Affinity Publisher. It exists as a 'component' of selected panels 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 on the Colour or Swatches panel.