Sims Turning Pitch Black When Wearing CC Made By Me

I recently (yesterday) discovered Sims 4 Studio and had so much fun creating so many custom outfits! However, I discovered that when my sims wear any of the CC I have created, they turn completely black, like a void. (see attached pictures.) However, I don't have this problem when using CC downloaded from other websites.

I have tried removing the Mods folder and adding the CC back into the new folder. My game is up to date, as well as the Studio. I repaired my game through Origin and tried batch fixing through the Studio (however, it only repaired the CC not created by me.) I also deleted the Sims 4 cache folder.

Any idea what could be causing this?? I'm a completely newbie to recoloring and using Sims 4 Studio so I'm at a loss on what to do! If it helps, I'm using GIMP.

imgur.com/a/xQp9W9y screenshots of the issue (with captions)