The two tails of the comets
The most impressive tail of a comet is caused by sunlight which is reflected and scattered by the dust trail of the comet - also called dust trail.
In addition, there is often a gas tail, depending on the amount of gas emitted by the comet. This gas is ionized (excited to glow) by the energy of the solar wind. This ion tail shines preferably with the emission lines of molecular Carbon (C2 at 511 and 514nm).
If you want a better contrast for the comet in the sky, you must consider the following when choosing suitable filters:
There is no way to show only only the dominant light fractions of the dust tail (as with emission nebulae e.g. in H-alpha) and to simply block the other wavelengths, because it shines in all colors.
Here we have the same problem as with the observation of galaxies - you can only try to block or reduce the general light pollution and to let the rest of the light through as unblocked as possible. Of course, with weak comets the brightness of the dust tail will also be weakened somewhat, but the sky brightening/light pollution is drastically more strongly suppressed. This increases the contrast and the comet stands out better against the background of the sky.
These filters work better for brighter comets, since they tolerate a dimming of their light better than e.g. dim galaxies. Of course, this only helps if the main part of the interfering light is caused by artificial light pollution and not by the atmospheric twilight shortly after sunset. Thus, the classic broadband Deep Sky Filter (Baader UHC-L) is perfectly suited for the observation of all comets, especially in the late night. Narrow-band line filters, on the other hand, should be avoided about the dust component.
Another part of the interfering light is the atmospheric airglow phenomenon and often the moonlight. Especially in dark locations far away from the city, a Moon&Skyglow filter ( 
Baader Neodymium (Moon & Skyglow) FilterBaader Neodymium (Moon & Skyglow) Filter (various versions available) with UV/IR blocker) can produce a better effect against these light sources than a pure Deep Sky filter. Such a contrast filter also provides a more natural image due to its more even spectral curve.
Especially in the bright twilight (also known as the Blue Hour), directly after the comet appears again from behind the sun (after the perihelion passage - when all comets are by far the brightest) it is especially important to dampen the high proportion of blue light in the atmosphere to increase the contrast. This is especially successful with the Baader FringeKiller filter. Most comets also have a blue-luminous ionized gas trail; therefore the broadband red filters often recommended for neutralizing the blue light will have no positive effect.
The 
Baader Fringe Killer Color Correction-FilterBaader Fringe Killer Color Correction-Filter (various versions available) , which was actually developed for simple refractors, has the advantage that it suppresses the blue atmospheric glow but still maintains full transmission around 510 nm, which are so important for the gas tail, especially because this filter does not attenuate the visual spectrum otherwise.
The 
Baader Semi APO FilterBaader Semi APO Filter (various versions available) , which contains the same glass substrate as the Moon&Skyglow filter, offers a similar, but even more contrast enhancing effect. However, an elaborate coating system additionally dims the blue spectral component and makes the dust trail stand out even more against the background of the sky.
With regard to the ion or gas tail, it is possible to capture these discrete lines of ionized gas via narrow band filters with maximum contrast, but unfortunately these emission lines differ from comet to comet. Theoretically one would need a narrow-band line filter individually tailored to each comet's spectrum. Thus such a filter would only be commercially available much too late - long after the passage of the comet. And it might not be equally effective for the next comet if the gas tail of this comet shines even in a slightly different range of the spectrum. The most probable, i.e. most promising, lines are the 500-514nm spectral bandwidth mentioned above. A wide OIII filter ( 
Baader O-III Filter (10nm) visualBaader O-III Filter (10nm) visual (various versions available) ), which is slightly open towards the red, is suitable for emphasizing the gas tail in these wavelengths. Since this Baader filter was also designed for the use at high open telescopes (such telescopes produce a blue shift of the central wavelength) it shows this also for comets favorable light redshift.
The advantage of an OIII filter is that it can also be used when there is no comet in the sky - namely to better see the OIII regions in supernova remnants and planetary nebulae. Of particular interest for comets is a SWAN filter ( 
C2 Swan-Band Filter (15nm ) – O-III parallelC2 Swan-Band Filter (15nm ) – O-III parallel (various versions available) ), which specifically allows the brighter emission bands of molecular carbon around 511 and 514 nm to pass, while blocking out the OIII line at 500.7 nm. The comparison between the OIII and C2 filters is particularly interesting, as it shows whether a comet tail contains oxygen. The SWAN filter specifically lets through the wavelengths that we normally expect from a comet, and which a broadband OIII filter only lets through by chance. This makes it even easier to distinguish comets that are dominated by a gas tail from those that are (still) dominated by a dust tail.
Conclusion
If we look at the above situation with the currently available filters, the following conclusion can be drawn:
The Baader Neodymium Moon&Skyglow filter is very well suited for twilight and for natural image reproduction - especially to improve the view of the dust tail. It would also be the first choice for gas-rich comets if it would not attenuate the carbon lines significantly at the same time. Nevertheless, it dims the main part of the comet light only very slightly and should therefore be especially recommended as the cheapest entry into comet observation with contrast-increasing filters.
Especially the Baader UHC-L / Ultra-L-Booster which was developed against light pollution in cities should have the greatest effect as a universal filter, especially after dusk. In addition to its strong suppression of light pollution near the city, it also highlights the above-mentioned gas lines of comets through its wide OIII window. In comparison to its predecessor, the UHC-S, the UHC-L was optimized tor light pollution by LEDs.
Not yet verified in practice (due to lack of large comets), the Baader Semi APO filter should be highly efficient, especially at dusk. This filter combines all the good properties of the Baader Moon&Skyglow filter, but in addition, it also cuts the blue range of the spectrum, i.e. the twilight interfering light. Photographically, the perfect blocking of all UV and IR parts of the spectrum is an advantage for all those who want to do comet photography.
The Baader FringeKiller filter (a technically highly advanced minus violet filter) is also suitable for twilight, i.e. the 'blue hour', because it only weakens this blue light of the sky and simultaneously blocks the photographically harmful UV and IR spectral range without harming the important OIII and cyan components of the gas trail in any way. As it only affects the blue component and does not block it completely, the colour balance is maintained.
All of the above filters are particularly suitable for either highlighting the dust tail or emphasizing the gas tail in the same way. The last filter in this list is only intended to emphasize the gas trail more than the dust trail.
The visual Baader 10 nm OIII filter becomes interesting exactly for the observation of the gas trail not only visually but also photographically, especially if you take digital pictures with different filters and then combine them on the computer.
The SWAN filter (C2 Swan-Band Filter (15nm) - O III parallel) is perfect for comets because it is optimized for the light of ionized carbon molecules, which is only transmitted by OIII filters accidentally.
Theoretically one could also darken the sky brightness with polarization filters, but of course, the position angle of the comets to the sun speaks against it. Therefore, polarizing filters will remain almost certainly ineffective.
The result of the above considerations can only be to react spontaneously to the respective situation and to have all filter options ready. Bright comets are rare - be prepared for all possibilities.
