Once you reach a certain karma threshold, you gain the ability to change the colour of the orange bar at the top of HN pages. The setting is labelled 'topcolor'.
Off topic, but why the heck does this require karma?
The required karma threshold is not much (500 if I remember), so it may be a way to say "thanks and keep going" to people who start investing some time to participate to HN.
It means if you change header colors of HN website nobody can guess that's actually HN.
#000000 is black in HTML. setting your top color black would make the text in the top nav invisible.
That is exactly the point. They don't want people seeing that they are on HN.
Right, I got that. I was explaining it to my parent. :)