A large problem with this whole idea is that there is no one single blue or green involved.
iMessage shades the bubbles depending on position, and shows both the blue and green bubbles as darker at the bottom of the screen, and lighter at the top.
That's definitely not iOS 7. Those screenshots are from a notched device, which means at least iOS 11, and based on the app drawer on the bottom it looks more like iOS 12 or above.
iMessage shades the bubbles depending on position, and shows both the blue and green bubbles as darker at the bottom of the screen, and lighter at the top.