How can I choose the number style or wheel style on my car?

For Sim-Stamped Number paints (the iRacing default), you can change the font and colors of your iRacing-stamped car number inside the iRacing UI under Paint Booth.

For Custom Number paints, a Trading Paints Pro membership option, iRacing won’t stamp a number on your car; that space becomes available to you to paint the number directly on the car in the style you wish.

Stamped car numbers and wheel styles (rim colors and finishes) are treated separately from custom paints, and must be defined in the iRacing Paint Booth, not Trading Paints.