Trev,
Probably worth while joining the Siril forum as your a bonafide Siril user and post your issue there
Not sure if there’s a facility to upload your data , that would be an advantage
https://discuss.pixls.us/c/software/siril/34
In regard to using overexposed luminance data , I’ve stacked and processed luminance data that’s almost a white out due to B8 City skies and I can still produce a colour image in LRGB using my post processing software Startools. As long as your individual data sets stacked correctly ( correct settings, aligned and calibrated ) and composited correctly together into a LRGB channel compositing tool or program together with being cleaned up using a gradient reduction tool in post processing then you should be able to produce a colour image.
Calibration should not affect LRGB colour. It should enhance your images.
Cheers
Martin