Let me add: you need to include ALL the chemicals added to the pool (by brand name and approx quantities), ALL the devices attached TO the pool, and ALL the special gizmos, if any, used on, with, by, near, or in, the pool.

Also, include a time line for the pool, from construction till now.

Waterbear is right: this sounds a LOT like metals. But fresh IS a "metal magnet", so if metals were in the pool, they might well be OUT now and ON your plaster instead.

The colors sound a lot like copper. Fresh copper stains on plaster will turn black if chlorinated just the 'right' way.

PoolDoc