I've downloaded a couple of unmarked european cars, but when they spawn for AI they're always white. To change the color I need to use the trainer menu, but ofc for the AI this is impossible. Is there a value I can change somewhere in OpenIV to make those vehicles spawn black as default instead?

I was wondering the same. I've been searching for nearly 4 hours and the only thing I have concluded is maybe a solid color skin/texture to put in the ytd file. I haven't found one though and also I could be wrong.