Wheels
I'm pretty sure if you buy ''aftermarket'' wheels from GT auto then next time you go back to GT auto there will be an option to return your wheels back to standard, HOWEVER, this will cost you the same amount as changing your wheels in the first place
Paint
When you buy or win a road car then the paint colour of that car will automatically go into your GT auto ''paint'' library.
Before you paint a car, go and look at it in your garage, because at the top middle of the screen will be the name of the paint colour.
The reason for this is because once you have hundreds of cars, when you go to your paint library in GT auto, you will find that you might have 15 blacks, 10 silvers, 20 whites etc and it's sometimes difficult to work out which one is the original colour. Knowing the name makes it alot easier.
If you spray a car and then dont like it, yes you can return it to standard, but it is not like the wheels, there is no option to spray it back to original, you have to find the original paint colour and then spray your car that colour.
Again, this will cost you, the same amount as any respray.