Most of the red colour that you see in photos of the Orion Nebula is due to H-a light at 656 nm. The greenish tint comes from OIII light at 501 nm.
If you look at the response spectrum graphs of cone cells, you can see that our eyes are very insensitive to H-a light (maybe 10% of L cones only) whereas OIII light will stimulate all three types of cones (S, M, L) at much higher levels. This is also why our eyes are more sensitive to green light in general - look at the response levels to light in the 490-560 nm range.
http://en.wikipedia.org/wiki/Color
(scroll down to the "Perception" subsection and look at the graph with red, green, and blue lines)