Something we're using as an extension in elementary OS and Pop!_OS is a "primary" color, and a "primary text" color.

https://github.com/cassidyjames/dippi/blob/5b22f996648bf49890b850814da62b4b5ebc4f55/data/com.github.cassidyjames.dippi.appdata.xml.in#L240

This allows clients to have a developer-picked accent color to use how they want in their UI, such as for headers or button accents. "Primary text" is used as a contrasting color when needed, and is planned to be validated for a minimum contrast ratio on the build server.

We do this rather than automatically picking a color from the icon (as GNOME Software does and Flathub used to do), as oftentimes a developer will want to pick a nicely-contrasting color rather than the same color as in their icon.

The exact implementation could be discussed, but I feel like this could be broadly useful as a way to allow developers a little more detail that clients could then use to spruce up the presentation of apps.