Hello,
At the moment, changing materials is the correct way to set outline color with code, and that was working the last time I checked... but maybe it's possible I missed something with serialization on an update.
Does your text look anything like "<m=blackOutline>Hello, <m=redOutline>world!"? The tags should be case-sensitive.
One thing that might be happening is... if you are creating the material tags through Unity's project window instead of STM's Text Data Editor, you'll have to click "Refresh Database" at the bottom of the Text Data Editor for STM to become properly aware that the material exists. (I should probably put a note on the inspector about this or change how this works to not be required at this point...)