One thing I noticed about custom schemes is that numbers aren't included. Like if you make a stock car paint scheme, you'll have to leave some space available for a certain number to be applied to your car. Probably good just to have a certain number applied to your car. However, what if you wanted to have your own custom number design, like if you run #57 and you want the 57 to have an American flag background to it?
While I still haven't played iRacing, it's one thing I noticed about certain custom paint styles in iRacing.