Similar issue.
If I modify the paint whatsoever (even another manufacturer colour) it reverts back to the original paint that the car came with after selecting “save to current car” and exiting the paintbooth.
They are aware of it. This glitch is on all versions of the game as far as I know, PC, Xbox One, Xbox One X (Both Xbox versions are the same), and has been present since launch. They are trying to fix it. Hope the fix is here with the next update in a couple of weeks.
I found a video on YouTube, someone having and showing this glitch.
He painted one car, choosing a normal color, the original color of the car is kind of Darker Green. The user painted the car in Green Grass, after that he chose other car and then went back to the previous one. And surprise, the car is once again Darker Green.
Might not be an obvious issue, or rather fix. I run FM7 on two Xbox one x and a one s, have customized many cars and saved them without issue. I haven’t seen the grey car issue in awhile. Which was only showing my car as grey, but could easily be corrected by selecting another car and back again, before starting a race.