This is an LHOORGB composition.

This M1 has a color that's kind of unusual, because of the filters. The RGB color images were taken with the photometric BVR filters in the Chroma Bessell UBVRI filter set.  This filter set has square-shaped filter transmission curves, rather than the bell-shaped curves of the most commonly seen classic Bessell UBVRI filters.  We switched from the classic to Chroma's new set to solve the problem of reflection halos around bright stars caused by the bell-shaped filter curves.  It works quite well, and the reflection halos are gone. Unfortunately this also leads to strange colors on emission lines. With this new filter set, both OIII doublet lines fall into the passband of V (green), and not at all in B (blue).  OIII typically has a cyan color when mapped with standard photographic RGB color filters or with classic photometric BVR filters, since it will be mapped to both the blue and green channels.  Under Chroma's new BVR filters, it's purely green, and looks very weird.

On the other hand, we also imaged M1 with Ha and OIII filters.  The narrow-band OIII image can be mapped to both the green and blue channel, so the OIII part in an HOO composition can have a cyan look.  Here what I present is an LHOORGB composition.  It inherits the green OIII color from the broad-band RGB image, but with a small shift to cyan caused by the addition of HOO. If I try, I believe I can nearly completely replace the green OIII (from the broad-band V) with a cyan OIII (from the narrow-band OIII).  But I decided to stay faithful to the data.  This is what the color is supposed to be when the filter set is the new photometric BVR filters from Chroma (plus some Ha and OIII), and I don't want to twist its nature.

For fun, I also uploaded two versions of different color schemes.  Rev.B is based on only the Bessell BVR filters (plus L), and you can see how green the OIII is here.  Here the color is calibrated with PixInsight's PCC (and SPCC gives similar results).  Only very minor tweaks on color were made in subsequent processing in Photoshop.  Rev.C contains strong contributions from the narrow-band HOO composition (plus L and RVB).  The OIII is mapped to cyan here, and a small fraction of Ha was added to the blue channel to mimic Hb.  The original (also the final) version somewhat sits between these two.