Hi,
White wool berbers generally go dark with a decent prespray and good agi. I tell clients that wool looses it opacity when wet, and will look fine when dry. Its not a lie because they do.
Its seems that ineffective hoovering leaves so much soil at the base of the floor and trapped under the loop, soon as you get it wet it looks terrible. Try a white towel when finished, if done right you wont get soil, but the carpet will look terrible.
Pre warn the client that this will happen and will get better when dry. Better you say it on survey or before you start work, then you look better.
Hope this helps
Graeme