I'm skeptical that it is from the batching...hear me out:
The second coat is usually shot on from the waterline down to a foot or two past the cove onto the floor. It would be exceedingly difficult to just shoot the top couple of feet of the wall and have such a clear delineation between batches. I also think the discoloration would show up as soon as they were done - the after install pictures look uniform.
That the discoloration showed up 30 days after filling is odd.
@Andrew Edward ... just to confirm - the pool has always been 100% full since completion and was never drained at all?