Here's a graphic of why it is necessary to pass OIII & Ha when taking images of the EC Nebula. I've used a Star Analyser grating to obtain a spectrum which clearly shows the strong emission of the nebula in those wavelengths. The zero order images of Eta Car and the nebula are at left, spectra to the right.
This explains the correct magenta-ish colour of EC Neb (red & blue combination), different to the bright reds of principally Ha-emitting nebulae. If you see bright red renditions of EC Neb it is either that the photographer has used Ha filters (filtering out OIII) or it has been colour-balanced or otherwise changed during processing.
Cheers -