What the water does show
Runoff can contain loosened dust, residue and material removed from accessible internal surfaces during the cleaning process. Before-and-after photos help document the physical result.
What it cannot prove
Colour alone cannot identify a mould species, bacteria or the cause of a person's symptoms. Those conclusions require appropriate testing and qualified advice.
The useful customer question
Ask what components were cleaned, how the surrounding area was protected and whether the system was checked after the service. Those details describe the work more reliably than the colour of a bucket.
The practical takeaway
Dirty runoff is proof that material was removed, not a medical or microbiological test result.