I'm confident that as the grandfather approached that wall of windows, his attention was totally on the child ahead of him rather than the color of the windows.
Many color blind people do not see certain colors at all, which can be dangerous and why our military tests all recruits for it.
JMO
Even if he not see the difference in window color, surely SA would have noticed the 1/3 of window (including the handle) still in the window frame. This would be an indication to a person with eyes that this is a window that has been slid open.