-PC signal (VGA)
-Video signal (VGA, converted from NTSC)
-Suppose the chroma key colour is blue
-three 2:1 multiplexers that feed either the PC or Video signal to the output
-A window comparator that measures the analog voltage of the RGB signals on the PC signal, and whenever it is green, it sets the mux address to route the Video signal to the output, otherwise it routes the PC signal to the output.
So the result would be that the PC signal is routed to the output whenever the PC signal is NOT green. Whenever it is, the video signal is routed. Hopefully the result is like a chroma key blue-screen-style overlay of the PC screen over the video.