I wonder if traces of boron might play a part too. It is a relatively
common light element and is predominantly green emission spectrum.
>
> And a blue ion tail, which is emissions from CO.
And carbon radicals of various sorts. The earliest spectroscopy of
bright comets was a little ambiguous eg.
https://www.nature.com/articles/009193a0
> And a pale yellow dust trail which is reflected sunlight.
Hale-Bopp had a particularly fine blue ion tail that was bright enough
to be perceived as a wonderful shade of blue with the naked eye.
I reckon we are due a decent bright comet soon.
--
Regards,
Martin Brown