Can you change the key color on macbook air...the black keys always look greasy.

You can theoretically buy each individual key cap in white, but at $8.95 per cap it'd be something like $500 to swap them all out. Unfortunately due to the design nature of the key caps, there's really no other way to do a transplant. Also, while the black may look greasy, the white will look dirty. Even if you clean and dry your hands before use (on your computer) your fingertips will naturally secrete oils and overtime the problem would just reappear.

A keyboard cover would potentially solve this problem, but there's no harm to the key caps as a result of these oils. A wipe down with a microfiber cloth once a week or so probably isn't a terrible idea either.

You can buy keys here. http://www.powerbookmedic.com/Macbook-Unibody--Air-Key-Replacements---Individual -Keys-Keycap-p-17302.html

And here is how to put them on http://www.youtube.com/watch?v=2Kg6EfuR-6Y

Chris