K.H.
I've also had good luck with OxyClean mixed in hot water. It will make it back to the original color tho (it works that well) so if the remainder of the carpet is discolored from time/use it will show up cleaner than the rest...kind of like an "anti-stain" but I am thinking you want the whole carpet to look uniform. So only try oxyclean if the rest of the carpet is still the original color.
K.