A new blue-green stain around a drain, a faucet, or a shower fitting after a treatment change is easy to dismiss as cosmetic. It can also be an early sign that the water chemistry has shifted and is acting on metal. Treat it as information, and gather the facts while they are still simple to read.
Know what the stain looks like
Copper and brass surfaces can develop a blue-green or greenish tint when the water is aggressive toward metal. The staining often shows up first in the sink drain, around a faucet base, or on a shower fitting. Note the color, the fixture, and whether the mark wipes away or stays. Take a photograph and keep it with your note so you can compare it later.
Watch for companion signs
Staining does not always appear alone, so check for a metallic taste, tarnish on brass fixtures, or a dull, rough look on metal parts that used to look fine. None of these prove a cause by themselves, but together they point to a chemistry question worth asking.
Ask whether chemistry was measured before and after
Ask the installer whether they checked pH, hardness, and other basic chemistry before the system was set, and whether they checked again after it was running. If the answer is no, that gap matters. Some treatment setups change pH or mineral balance, and a provider should be able to explain whether the change could affect the metal in your plumbing.
Ask what adjustment, if any, was specified
If the water is aggressive toward metal, some setups include a neutralizing step or a different media choice. Ask the provider whether one was considered, whether it was included in the quote, and what it would cost to add later. Check that any such item appears in the written scope, since a missing line item is easy to overlook at install time.
Act early and keep records
Keep the photographs, the dates you first noticed changes, the fixtures affected, and the answers from the installer. If the staining spreads, or if you see pinhole drips or leaks at a fitting, stop assuming the cause is cosmetic and bring in a plumber. Early notes give the provider something specific to test, which is much faster than starting over from a vague complaint.