My understanding is this: a page will display visited links in different colors. The javascript can then query the color of any given text element. Hence, you can create a hidden page (iframe) that can check to see if you've visited specific web addresses.
This is something that should be of great concern to all FReepers - it is very easy for, say, a government website to check if you've been to http://www.freerepublic.com! (If that link is discolored - which it certainly should be for this crew - then javascript can detect you've been visiting it....)
But isn’t it the browser that is the only thing determining the color? In this case, I believe it is only the browser doing it. There’s no reason for a browser to SEND all of our history to others to simply change a color on only our screen.
I think other history-related issues are at play with this thread and not the hyperlink color thing you mention. Sorry.