Toning - A term used to identify the corrosion or breakdown of the metal used in Hot Wheels cars.  Not all dark areas under the paint is toning.  Dark areas can also be caused by imperfections in the casting that show through the paint, such as swirl marks, tiny dark spots or "hot" spots, fogging or even just a dark cast.  Toning can worsen over time if proper storage is not followed.  Darkening (toning), under the paint takes many forms as web toning, marbling, hazing, mottling, etc..   Toning is caused by lesser quality metals used in the casting. Corrosion is different than toning.  It is normally identified as a white crusty form building up around the casting.