See the picture attached.
To me the color looks like they are organic but it covers a large portion of the whole pool. The pool does use copper pipes and the owner "maintains" the water himself.
What do you guys think? Buy jacks stain ID kit?
I think the pool is pretty old but the plaster appears to be in good shape, besides the staining obviously.
UPDATE: Ok so they said it wasn't painted. They also said that the neighbor said that the pool was drained and refinished in the 80's, and the pool was originally installed in the 60's!!!