When doing astrophotography, only use RAW -- avoid JPEG. Astrophotography images require a lot of post-processing and JPEG images don't have the adjustment latitude (JPEG is mostly for "final output" -- when no further adjustments are necessary).
In RAW, there is no white balance (the white balance is simply recorded as a preference in meta-data but is not actually processed in the image. Post processing software usually ignores it ... but some post processing applications will attempt to use it.)
The comet appears green because it is green.
As the comet (or any comet) "melts" as it nears the sun, it gives off a lot of dust, but much of the ices are molecules that have very low melting points (e.g. CO2 ice melts at roughly -57°C (rounded value)).
These gasses absorb ane re-emit electromagnetic radiation and this causes them to "glow". The colors you see are based on the elements involved.
Here's a shot (mostly unprocessed with the exception of slightly adjusting the background) that I captured Friday, July 24. But even though binoculars, the "hair" (coma) around the head of the comet is distinctly "green". (I did not want to post a processed shot because that can be misleading).

Image captured using Canon 60Da using 200mm at f/2.8 and ISO 800. The camera was on a polar-aligned tracking mount (Losmandy StarLapse)
Note the very strongly 'green' coma around the head of the comet. This is not an artifact of the camera ... it appears this when observed through binoculars.