Thanks Jim!

The false color is an attempt to simulate real color images using the 3 popular narrowband filter set; the combination requires a little pixel math in Maxim or similar software

Red = Ha+0.33*S2 (requires a rescale back to 16 bit)
Green = O3
Blue = O3+0.33*Ha (requires a rescale back to 16bit, statistically Hb = 0.33*Ha .... so a Hb filter is useless if you have already taken Ha)

You have to perform a DDP much like you described in your M81 post, with the trick to best align the histograms such that the peaks line up.

Tim