We haven’t posted it on Meta yet (will next week) but you can override all colors in a theme in your theme’s about.json file (see the color scheme section of Developing Discourse Themes & Theme Components)
You can override $primary and other colors as mentioned in the post, but you can also override secondary colors like $primary-low, $primary-high, etc… You can see all the color variables listed here https://theme-creator.discourse.org/styleguide/atoms/colors
Note that at the moment this level of color overrides is only available in remote themes, not themes created locally in the admin ui.