So I tested a bunch of different models and couldn’t replicate the issue. You’re using a relatively recent version of the program (last updated about a month ago) and the material files are named correctly, right?
Viewing post in 3D Model Converter for GameMaker comments
So tested it on another computer and the issue still presisted. However loading in a model from kenney's Retro Medieval Kit worked fine, the texutres loading in without issue. Can Penguin just not load colour from mtl files?