Stain colour
Colour on drywall is a hint about the water’s path, not a chemistry report.
Green-blue marks often show up under copper lines. Brown marks often show up under old moisture. Neither colour lets you skip investigation.
Photograph colour before anyone primes.
Primer erases the only hint you had.
Do not scrape a sample into a bag as a hobby.
You can make a hole without a source.
Match the stain to a pipe run if you can see one in a closet.
A green ceiling under a copper closet is a useful sentence.
Questions people ask
Does green mean pinhole for sure?
No. It raises copper as a suspect. Confirm with a visit.
Should we test the water?
Building water chemistry is an assessment question when pinholes repeat. One stain does not require a lab kit from a hardware store.