From the type of error it shows, I have a feeling it's a configuration error in the plugin parameters. Did you make sure all the parameter fields are properly initialized and have correct values? Because the type of error seems to be related to colors.
Do me a favor so we can solve this quickly:
Contact me via direct message: https://undermax.online/undermax/support.html
And send me a copy of the demo (one that I can open with the RPG Maker MZ editor) of the project you're using with the current configuration. That way, I'll be able to open it on my setup to run more specific tests and we can resolve this more quickly.
I'll be waiting!