Sketch: how to create colour palette in symbols page

I'm new to the Mac app Sketch, and I've been watching a few tutorials where a color palette (a grid of colored rectangles) is created on the symbols page, and a color from the palette is assigned to an element of a symbol, meaning that whenever the color is updated, all instances of the symbol have the element's color updated.

I want to know if a color is a special symbols page component.

In the example above, how is the link between the color in the palette and the element in the symbol made?

What do I need to do to be able to select a color from the palette on the symbols page, and be able to assign it as the color of an element on my canvas?

Help appreciated.