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 Developer’s guide to Discourse Themes)
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 Discourse Theme Creator
Note that at the moment this level of color overrides is only available in remote themes, not themes created locally in the admin ui.