Is there a way to exlude favorites from a search? I've tried "-fav:USERID" but that didn't seem to work. Alternatively a way to just highlight favorites, possibly with a userscript, would also fulfill my needs.

It isn't currently possible with our search, but it's already been requested in the trac. For technical reasons, it might be a while before we can implement it n

Putting a colored box around posts you already have favorited probably wouldn't be that hard, but it would run into the same problem that the favorite search does - the favorites table and posts table aren't compatible, so getting them to work together requires a hack, and this hack puts a big load on the server. This is why there's a maximum number of favorites before you get blocked from searching within them. A single user could lock the server up for long enough for everyone else's requests to time out from just one favorites search if the user whose favorites they're searching has too many.

A userscript could probably do it, but I don't know of any that do.

There is a way that I figured out, but it's still quite convoluted gelbooru.com/index.php?page=forum&s=view&id=8851