Have a read of this and you'll realise why you're seeing a bit of green and no red. (but also why everyone with a camera gets worried if their M42 isn't "normal" and showing lots of red!
)
http://starizona.com/acb/ccd/advtheorycolor.aspx
Seriously though, it's really tough for any astrophotography to be "realistic" because of these difficulties. Many people argue it's just "technical art". Hope this helps with your visual interpretation of nebs too.