Iphone problems with back glass

Hi again. I talked before about the speck under the back glass of iPhone xs in the right corner. I was thinking a lot, so I decided to watch videos about replacing parts for iPhone. Back glass for iPhone xs on the other side(where it has to be sticked) is black. But if you look better, on the edges is normal color(mine phone is silver). I think when was the iPhone on the manufacturing process that they didn’t coloured very well that point of glass. Can someone tell me if I’ m making a mistake??? Thank you...