My experience is similar to many others here. Something I noticed that seemed obvious AFTER I sorted it out is if I use my 18" and am way zoomed in on Orion I see gray. If I use a much wider field of an 8" short scope I then easily see green in Orion nebula.
Planetaries are easy to see color in generally.
So my thoughts are if you are zoomed in a very faint green is 'white balance' corrected by your brain to 'gray'. If you have a very wide field and some white stars are in the field THEN the really big nebulae look green (if a ton of OIII is present) or pink if much more Ha (really strong Ha) is present.
The surface brightness of a bright PN being maybe hotter than the faint nebula make it easier to see the OIII and thus the green or sometimes Blue.
So I try to find a few stars just outside the nebula and get my brain 'color balanced' to that color THEN swing back in over a wider field of a big nebula and maybe if your 'color correction' is slow you may see the color better.
Sometimes our brain gets in the way of seeing faint colors is the point.