60-30-10 rule
A balance guide: 60% main color, 30% secondary color, 10% accent.
Also called 60/30/10 rule
The 60-30-10 rule suggests using a dominant color for about 60% of a space, a secondary color for 30%, and an accent for the last 10%.
It comes from interior design but works well for websites and slides: neutral background, a supporting brand color, and a bright accent for actions.