Many mobile phones will allow you to remove the cover (or fascia) and replace it with a new one. Fascias are available in plain solid colours to smarten up a scruffy or tired looking phone, or a vast range of fashion, slogan and picture covers, to add your own individual stamp. Keypads are also available in a selection of metallic, gel or crystal styles.
However, you should be aware that mobile casings can often play a part in the phone's operation: some models have metallic coatings inside them to improve RF performance. Others have internal antennas, and a metallic case will seriously degrade their performance.