Hi David,
I did test subs in narrowband and the signal is surprisingly strong in SII, which is deep red. Ha is obviously strong too (red again), but not that much in O3 (green)- just around the central section of the nebula. So it might be a bit of a challenge to show other colours than red in an RGB image - but if the colour is not really there then let's not artificially force it into the image
EDIT: To my eye your image is heaps better than this one:
http://apod.nasa.gov/apod/ap030717.html