The theme uses 3 colours: Primary, Secondary and Tertiary – these colours can be modified in the Customiser.
The Primary colour is used for headline text, the footer bar and other page elements
The secondary colour is used for buttons and table backgrounds.
The Tertiary colour has no pre-defined use but, along with the other colours, is available in the block editor Colour Palette for setting text and background colours for some blocks.
You are not restricted to the defined colour palette and can click on Custom Colour to select any colour. However, to maintain consistency, it will usually be better to keep to the defined colours.
For more information on the benefits of a theme-defined colour palette, see Why your WordPress theme should use a custom colour palette.
Also see WordPress Customiser.