Roof panel discoloration or etched

Yes, please try to send a photo of this to me at todd@asktoddmiller.com Thanks.

I have a brand new galvalume silver metal roof. Roofers started yesterday and finished today. I watched them off and on. While they were working on it, it looked and still looks beautiful for the most part, with an exception. The roofing contractor himself has not yet come by for the final payment. I didn't give it the once over before they left as I had watch them so much but after they left I walked around and got up on the my neighbors porch as I had done before only this time I noticed an obvious discoloration or stain (hard to say even with my binoculars) what it is. One area starts directly under the very bright and shiny Gooseneck Vent and the discoloration appears to be a running in an uneven pattern going about half way down the panel in the 2 flat sections. While my roof is silver, over all its appearance is more whitish. However the so called stain area actually looks silverish. I had looked at that area many time while they were working on it and that section looked just like all the other panels whitish. Only now that panel shows silverish streaks wide and thin and standsout against the other panels. The panels, ridge cap,drip edge, underlayment and screws all were dropped off by the Gulf coast metal manufacturer. However the gooseneck vents and flashing were brought by the roofer. In previous research on metal roofs I did remember areading something about certain metals not compatible against each other wondered if this was the problem. One of the roofers at one point near the end of installation had ask where my water hose was as he wanted to use it on my roof. I never asked why just thought it was how things were done. They did have a blower up there and used it at times to clear leaves. Don't know if they might have spilt something on that panel accidently causing it to etch the clear coating and maybe thats what they tried to rinse off. Just guessing at this point. The contractor is coming tomorrow to pick up the final check and I will ask him about it but need to know how I should handle this conversation. I'm concerned he may dismiss it with some excuse and don't know whether he'd be right or not. See I am a woman and in searching out different contractors I have not been treated as if I can comprend the science of roofing even thought I spent 15 years in the building business as blue print reading, take-offs and cost estimation for two different lumber companies. While I have not delt directly with roofing with the exception of trusses I still have a good working knowledge base of home construction and did alot of research before going forward with metal. So if you could enlighten me please I need to know if this might be something minor that can easily be repaired or should I insist on panel replacement and if so will they be able to do that now that the roof is completed. I didn't think to get a pic of it while it was still light out but I will try to get one tomorrow morning if i can.