Hello,
The Adventure Creator plugin was developed by a friend of mine if I'm remembering correctly, so I'm also unfamiliar with it! (And it hasn't been updated in a long time... I think!)
But going by that code, that should be correct for setting a baseline colour for a SuperTextMesh object... Are you able to edit the script attached to the "STM SubtitlesUI" prefab that you mentioned and add that code to it? Look for wherever that code is setting "superText.text", and you should put the line of code you shared on the line before that, so the colour is set before the text is set. My thinking is as follows: the script has to be getting the dialogue from somewhere, so that would be the right place to also grab and set the colour data.
I'll be able to take a look at the code myself later, (things have been really busy for me, sorry!) but this should be a pretty easy feature to be able to add yourself!