Have this spot about 1 1/2" dia. at in-laws house...tried a few times and it just won't go away..doesn't even want to fade..I don't know what it is and neither does my sis-inlaw....rest of carpet cleaned great and all the spots and stains are gone except for this one..tried pre-scrubbing using brush...tried fiber plus pad...glad pads...100% cotton...enduro...used peroxellent with spot and boost...used intensity red...tried a feww other solutions and mixes...nothing...only other trouble spot, I was able to fade maybe 90%..but this one has me stumped.
Some staining materials which end up on carpet can permanently change the color of the carpet. One example is pet urine, which can result in permanent yellowing of an area. You can scrub all day & night with all kinds of products, without removing the new color. It is what it is.