particulary 727s -- obviously easy to notice -- see red paint in below pictures? -- I am curious -- I have seen blue, green, black --- especially on the left side of the plane (when looking at the plane from outside) -- why is that?
The right hand sliding window on this aircraft can be opened from outside in case of emergency. If you look carefully you´ll see the handle below the window (painted red with placard below). The red color of the window frame helps to identify it as an emergency exit for the rescue teams.
This feature is an optional installation on B737´s too (only the r/h side).