Just to add....The only time I've seen real browning was 12yrs ago on a job where a 2lt pump up was cracked and emptied its content ( a high ph prespray) in a puddle on a wool carpet with a jute backing. Got a call back 2 days later about a brown circle.I crapped my self but I was in the NCCA so called Paul Pearce who talked me through the remedy
Browning occurs when you overwet some natural fibres either on the face or in the backing which releases a brown 'dye' ( I use brackets as I don't think it's an actual dye)