Shaun's reply above is spot on!
I had exactly the same problem many years ago when I was still green. It was a commercial flocked carpet (well it was flocked when I was finished with it!

) made up of little red and blue squares like a chequered pattern.
In the traffic lanes, after extraction they looked filthy... but after a closer look there was simply no red fibre left at all! The water out of the machine looked crimson, but I realised it was all the little bits of fibres floating in it. Luckily the building owner accepted it was the carpet not me.
Have seen it on BW's too some times, and occasionally on plain polyprops by a patio door facing south - you just get 'dust' off a plain carpet and maybe don't notice it but if you look carefully you can see the degradation - the pile will be a bit crumbly too.