Your finish should be covered by at least one warranty from PebbleTec, and possibly a second one from the installer. What does the PB and/or his PebbleTec installer say about the color issue? Have they yet? Did you already get the ol' "Oh, it'll go away on its own." spiel? Ask them to arrange a warranty review directly with PebbleTec. The installer should have been certified by PebbleTec to apply that material, and PebbleTec is suppose to stand by that work. If you've got a two-tone pool, that is not a standard installation. Have PebbleTec weigh in directly about this issue. They've been known to send their own rep to inspect a problem like this.
The PB and his installer are probably not going to volunteer this step, because if it is determined they are at fault, they'll be on the hook to make it right. If they won't arrange a review by PebbleTec, then call PebbleTec yourself.
If there has been any mention of an acid wash, put on the brakes immediately and check back with us. That's a no-go.
Who has been taking care of the pool chemistry for this first month? That's important. If it was the PB or his staff, you're in the clear. If it was you, you might find the PB and/or PebbleTec will push back and claim it was bad water chemistry that caused the problem, and so your fault, and so no remedy. Did the two colors develop over time, or were they like that from day one? Can you prove that? If you were taking care of the water, did you test the water correctly, and keep records of the test results? Unfortunately, if there is to be a dispute about fixing the color issue, you'll need to be prepared to prove it wasn't your doing (if in fact it was not).
No need for things to get ugly just yet. First see if you can involve the PB, the installer and PebbleTec to come to some kind of reasonable resolution. That starts with a call to PebbleTec.
At some point you'll need to decide what a satisfactory solution is, and to what extent you want to fight for it. Can you live with it? Would it be more palatable if a discount was offered? Or do you want the finish replaced, and if so, are you willing to endure what that will take to get done. At first you'll want to know what the PB and PebbleTec are willing do. Maybe they'll offer to replace the finish. If not, then you'll go from there...