There seem to be curved, gray "lines" throughout the picture, like this one in the bottom center. Perhaps this confuses your eye, since the resolution of anything outside the center of your vision is terrible and mostly your brain's best guess? Also color vision gets progressively worse due to fewer and fewer cones/rods (I don't know which one was which) outside the center of your vision, so maybe your brain literally cannot properly distinguish the green lines from the gray?