Very intricate detail Stefan. Great image!
As for the colour. In theory Hb gives exactly the same regions as Ha, but at about 30% of the strength, so according to some people it's pointless taking Hb
This is what Don Goldmann says about H beta: See Qu5
http://www.astrodon.com/Orphan/astrodonfaqnarrowband/
Richard Crisp suggests blending some Ha with the OIII to make the blue channel. See here
http://www.narrowbandimaging.com/synthetic_rgb_page.htm
Geoff