As per the Tarantula image, the OIII mask background was normalized relative to the Ha mask giving a more accurate account of the OIII and Ha distributions in the image.
Strongly cyan indicates the OIII emission is dominant, blood red Ha dominant.
In between colours indicate OIII and Ha in different ratios.
http://members.iinet.net.au/~sjastro/etaC_Ha_OIIIc.html
Steven