I see the gray of M43 and nearby NGC1977 as bluish-gray. These pics tell the story as to why. M43 dominates in the HBeta emission, and NGC1977 is a reflection nebula (at least part of it is), so both are dominated by the blue.
Mauro has a good point about greens in the Huygenian region. I've always seen this as a gray, with a very pale greenish tint to it. In a 32" scope, the reds dominate for me and the greenish tints in M42 nearly disappear unless an O-III filter is used.
What is interesting to me is that I have always seen one "lobe" of the nebula (one side of the central region) as somewhat "peach" colored as opposed to the "dusty rose" of the other lobe. The lobes are the section of the nebula inside the "ring" and separate from the "arches" or "wings".
Peach is a combination of yellow, red, and gray. I suspect, after all our conversation, that this may be an illusory color.
As for the HAlpha test, the weather forecast for SoCal for Thursday through Tuesday is clouds and rain. I may not get to test the filter this weekend, but we will see.
As to why I call M42 "The Diamond ring", see this photo: http://apod.nasa.gov/apod/ap040927.html The diamond is the Huygenian region and the circlet goes all the way around, as you see. At low power in my 12.5", I always see the complex similar in extent to this photo, but with less contrast, and not, of course, with the vivid colors shown in the photo.
-------------------- Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie