To make the star trail images look as colorful as possible, I recommend 3 things:
1. Set appropriate capture parameters so that the stars are not too burned out and keep enough of their color. I usually use ISO 800 (or even lower) and close the aperture 1 or 1.5 stops (so that the aberrations of the optics are reduced and the stars are perfectly sharp).
2. Shoot in RAW and then stack in Nebulb the RAW directly. I never work with JPEG because, among other things, they do not allow to recover any color from the brightest area of the stars.
3. Do not activate any noise reduction parameter in Nebulb (unless you have too many hot pixels).
In case the stacking you do in Nebulb is specifically for a star trails image (like a circumpolar one) it is not recommended to activate any of the noise reduction options in the “Preprocess” section.
These two settings manage to reduce the noise (especially the hot pixels), but the problem is that the stars (being very small) lose much of their color.
In any other type of photography (Milky Way or meteor showers) this is not at all noticeable because the stars are small.
But in the case of star trails images, if the stars have less color the resulting trails also have less color, resulting in much paler star trails images.
For that reason I do not recommend you to activate any noise reduction setting (especially color noise) when stacking star trails.
The “chromatic aberration removal” and “vignetting” are no problem to apply if you wish.