Didn't do anything else tbh. Created a new team, did the materials, edited the json (I edited the original one as well before I made the new team)
I found that you need to delete the original alt.dds, json and region.dds files once you've got your own files in the virtual ride folder. I've just spent the afternoon writing a step-by-step tutorial for novice GIMP users and my vehicle did exactly the same thing on track (original skin instead of my own) until I removed the auto-generated alt* files. I have a new problem in that the materials break though....
somebody posted the original materials showing through was solved be editing the region file.(not sure what that is)