Colour and Contrast: How Designers Build Hierarchy

Hierarchy tells the eye where to go first. Size and weight do much of that work, but colour is the fastest signal of all.
Modernist designers often worked in black, white and one accent. Restricting the palette made every use of colour meaningful: a red word or a single coloured shape became the entry point to the whole composition.
Josef Albers later showed in his studies of colour interaction that no colour is seen in isolation; its neighbours change how it appears. Contrast is therefore relational, not absolute.
In practice: decide what matters most, give it the strongest contrast, and let everything else step back.