If you want to debug it try running the tool from a Command Prompt instead of Opus. Also note that you have to restart Opus to see the changes, as mentioned in the root post. (And make sure you are really restarting Opus; not just closing its windows and re-opening them. See here.) Beyond that I don't know what to suggest as I didn't write the tool.
All it does is set the folder icon, though. You can do that from the standard Properties dialog (see here). You can also set the folder label colour via a simple built-in Opus command (see here). If you're having trouble with the approach in this thread, try the others as they are a lot more simple (and don't require restarting Opus to pick up each change).
Edit: New version 2 of the tool no longer requires restart so I've crossed out those parts to avoid confusion in the future.