CSS lets you color links based on whether the user has visited them or not.
Most of the Asteroids in the game are links - they use those same CSS rules to color themselves based on whether you've visited them or not.
Script on the page can't tell what color the asteroids are, but it can tell what asteroids you've clicked on. By making the visited-link color the same as the background color of the webpage, we get the user to give away the color of the links, because they'd only be clicking on the links that they can see - the ones with a color different to the background.
Thank you, that was a much clearer explanation of what was happening. Also explains why I'd occasionally see a black circle drift over one of the colored ones.
Real life example - back when the CSS history snooping trick still worked, a travel site I worked on used to offer additional discounts to people who'd visited competitor sites that we knew were beating our regular pricing.
Imagine you had a brilliant idea to start a company which served demographically targeted advertisements to users across the web. Since you are brand-spanking-new you have very little data about users. You need to find a way to quickly and cheaply build demographic data about users so you can actually accomplish your mission.
If you launch a ton of these types of games, you could build a pretty decent database of user's browsing histories. If you are using news sites as your sites of interest, you could use that browsing history to build up demographic data about each of the people who play your game (demographics for large sites like CNN are pretty well known).
There are certainly other reasons, but this is the first one that came to mind.
Most of the Asteroids in the game are links - they use those same CSS rules to color themselves based on whether you've visited them or not.
Script on the page can't tell what color the asteroids are, but it can tell what asteroids you've clicked on. By making the visited-link color the same as the background color of the webpage, we get the user to give away the color of the links, because they'd only be clicking on the links that they can see - the ones with a color different to the background.