One of the very first paid jobs i did,was a hallway,pale green woolen ,had had a lot of traffic over it`s life and never been cleaned.
Did vac very through,my equipment was all brand new,so working at it`s full potential,soon as i put soloution on it,it went blacker than black,custy was stood behind me watching
And said i hope that goes back to it`s niormal colour !
Oh yes it will all be ok,many wet passes later,i got it out !
That`s when i learnt to check depth of soiling in the carpets whe3n quoting,what should have taken half hour,turned into 1.5 hrs !