In order to prevent star colours being skewed during the mapping process, the stars were removed from the OIII and Ha masks.
http://members.iinet.net.au/~sjastro/etaC_Ha_OIIIc.html
I have tested this process on NGC 1955 which is strong in Ha and weak in OIII to see if the colour balance was preserved without the creation of "false" OIII colour.
No OIII emissions were enhanced in the process thereby resulting in a correct colour balance being preserved for this object.
Clear skies
Steven