The first method:
Spray Paint with white background
The paint stripping off after few weeks of usage. I use Kyrlon spray paint for plastics bought from Ace Hardware at RM42.00. It's not really easy to use because, it's spray! First you have to use it out door and secondly, you must be pro to use it or else you will cause uneven pain distribution like picture above, that later i have to sand it off. Very time consuming method and not so effective.
The second Method
Spray with letter
This method even worst. The lettering is not as sharp as original. After a while, it will be strip off by your fingers because the paint not embedded with your plastic keys effectively. Dont even think of writing it with white color paint or pen. Trust me, your keyboard will look like a cheap keyboard later. You can refer to link below and you know what i meant.
Vinyl Lettering Stickers