Siril has the ability to demosaic the FIT file, there is a checkbox on the file conversion tab. I found it best to de-mosaic the pre-processed / stacked FIT file as a last stage, it speeds the processing up on the intermediate steps as you are working on one layer not 3 (Red Green and Blue).

The siril documentation explains this more fully.

Here's the method that worked for me.

To process a star trail sequence with multiple dark frames in siril.

CREATE MASTER DARKFRAME.

In File Copnversion tab click change dir and add a working directory eg "siril working dir". Add raw darkframes in file conversion source and then enter sequence name eg darkframes, press convert button. We now have darkframes_seq loaded. In Stacking tab select Stacking Methods > Median Stacking and Normalisation > No normalisation. Then click Start Stacking to create darkframes_stacked.fit

CREATE FIT from RAW.

In File Conversion tab clear source with remove all files from list button then add raws to be processed in file conversion source and enter new sequence name eg P6174240-53, press convert. FIT image files are created and sequence P6174240_53_.seq is automatically selected.

PRE-PROCESS SEQUENCE WITH DARKFRAME.

In pre-processing click use dark and browse to darkframe_stacked.fit then click Start pre-processing. Pre-processed FIT files prefixed "pp_" created and image sequence pp_P6174240_53_.seq is automatically selected.

STACKING SEQUENCE.

In Stacking select Stacking Methods > Pixel Maximum Stacking and click Start Stacking to create the ingle stacked image pp_P6174240-53_stacked.fit.

DE-MOSAICING AND IMAGE PROCESSING.

Check Demosaicing then use Siril Control Center File > Open > pp_P6174240-53_stacked.fit. to carry on in the same window.

In Control Center click Image Processing > Histogram and apply autostretch or alter sliders. Control Center > Image Processing > Remove Green Noise > apply. Control Center > File > Save As > Tiff. Enter file name eg debayered_pp_P6174240-53_stacked.tif and save. Open in Gimp (ideally 2.9x other editors are available) and adjust as necessary (curves / saturation / cloneing) then export.

see https://free-astro.org/index.php/Siril:Manual and https://free-astro.org/siril_doc-en/#Reference_documentation_1