Some folks use white eyeliner to line their lower inner lid ("waterline") with the theory that it brightens the whites of your eyes. A better trick (in my opinion) is to use a flesh-toned liner for that, but I digress...
I have also seen a lot of white liner usage over colored eyeshadow along the top of the lid. Just treat it as any other liner, and use it to hilite or provide stark contrast with your eye look. It looks super cool, when done right.
I personally think that WoC could use this either way, but again: I find flesh-toned liner in the lower waterline to be a bit more subtle.