Can leaves left on a car stain the paint?

Yes. Wet leaves, particularly in autumn, release tannins and other compounds that can leave brown or yellow leaf-shaped stains, and decaying leaves trap moisture against the paint. Light colors show this most clearly. Remove leaves before they dry onto the surface, and pay attention to the cowl area under the windshield, where they collect. Light staining may wash off; persistent marks may need a cleaner polish.