We "LOVE" our cats (have 4) but "HATE-HATE-HATE HAIRBALL RESIDUE"!!! (did I say "HATE"???)
Well...while reading some cleaning tips on vinegar, it said that it also removes stains from stomach acid. I mixed a solution of 1 to 1, of vinegar and water in a spray bottle. Sprayed the residue and saw it instantly become lighter in front of my eyes! Let it sit for awhile and took a soft brush, luke warm water with a drop of liquid dish soap mixed in it and scrubbed it clean! NO MORE STAINS on our "light tan" carpeting!!! Give it a try...IT REALLY WORKS!!!