On Monday, December 15, 2014 9:42:59 PM UTC-7, wrote:

I have a computer keyboard with black keys, and white letters on them.
Several of the letters have worn off the keys. I'm wondering if there
is some sort of white permanent marker to use to put the numbers back,
or something else?

The very best place for information about every aspect of computer
keyboards:

https://GeekHack.org

Regular labels placed on the fronts of the key caps will last a
long time.

Instructables.com has an article on restoring the factory look, but
it won't last long unless you coat the tops with something clear,
and even that will wear off:

http://www.instructables.com/id/Rest...Keyboard-Keys/