Bruce, The human eye processes the info received immediately, unlike a film or ccd. Therefore to actually 'see colour' we need a fairly large aperture t'scope to gather the avail photons. The cones are the cells that do this. ( cones = colour).
With a larger t'scope, >8", you should be able to see a faint green glow in M42, poss a bit of pink to very faint red. Age becomes a factor as well.
I can see green with just a hint of pink in my 10" dob & @ 60 yrs, I think that's great. Conditions have to be pretty good too.
L.