The most commonly used paint for painting keys is acrylic paint. Acrylics are water-based, quick-drying, and highly pigmented, which makes them perfect for decorating small surfaces like keys. They also adhere well to metal and other materials commonly used in making keys. Additionally, acrylics are durable and can be sealed with a clear coat to protect the painted surface from wear and tear. Other types of paint, such as enamel, oil-based, or spray paint, can also be used, but they may require additional steps or materials to properly adhere and dry.