The Apple wireless keyboard bundled with iMac or the optional wired Apple keyboard with numeric keypad are neat and good in design. They are in good match with white keys on the anodized aluminum enclosure despite it is not be as good as third party keyboards for typing. However, keeping the keys in good looking is a headache. As time goes by, dirt or grease from fingers are attached to the white keys making them yellowish looking.
There are protective cover to put over the keyboard. If not, you can buy a pack of melamine foam cleaning sponge such as those from Daiso Japan which can be bought in Aeon Store in Hong Kong. Rubbing with a slightly moistened foam may remove otherwise “uncleanable” external markings from surfaces. The foam wears away, rather like a pencil earaser, leaving behind a slight residue which can be rinsed off. It is tested it works effectively to clean the keyboards!
Although it is effective, melamine foam contains melamine which is described as being “Harmful if swallowed, inhaled or absorbed through the skin. Please use with necessary precautionary measures.