That filter bar is a feature of my browser extension. You already can filter by tags and by color, without any special prefix: just type the tag name or the color into the filter box. (There is a bug with regards to tags with capital letters in them, which currently can’t be filtered on. I’ll release an new version soon with a fix. [EDIT: now fixed in the latest version.]) Note that the official Beeminder name for the color between red and blue is “orange”, even if the shade is somewhat yellowish.
The suggestion of “done” and “open” meta tags is a good one. I’ll see about implementing it.