For some reason, the occasional word will be in red, especially in old threads, especially with usernames. Any reason why? Was this an old tradition that died out, or programming weirdness, or what?
You probably came through a link that somebody got through a search. If you add &highlight= and then some word to the end of a thread URL, it’ll highlight that word in red, like so.
A link would be useful, but in general… If you’ve used the seach to find a thread, your search terms will be highlighted in red when you open it.
Other than that, users can specify colors to be used in their posts, it’s just generally frowned upon as being unuseful in most instances.
Do a search on posts by the Grapist for examples.
ETA: Simulpost, what he said.
What is nice is that you can do it on the fly:
http://boards.straightdope.com/sdmb/showthread.php?t=493616&highlight=link
Click that link and every time the word Link occurs it is in red.
You can also have more than one word highlighted.
http://boards.straightdope.com/sdmb/showthread.php?t=493616&highlight=link+search
For Link & Search to be in Red.