Like some bad joke this cannot be done through the Settings app but only the Registry Editor. Here I will explain how to do it and provide a File for easy application.
First, pick your favourite accent color and deactivate “Show color on Start and Action Center” and “Show color on title bar” in the personalization section of the Settings app.
Then open the Registry Editor and navigate to
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\DWM. Once there, set ColorPrevalence from 0 to 1.
AccentColorInactive to the title bar colors of your liking in classic hex format, e.g.
ff7f00 for bright orange.
Here is a registry script for easy reapplication when you decide to change your primary accent color through the Settings app: https://x.ezl.re/windows10titlebarcolor.reg Don’t forget to change your colors accordingly here too!
Thanks to Chaoss86 for the explanatory reddit-post.