In terms of colours for Exegol, I’m firmly in the camp of:

A greyscale planet, with no tint, to imply absolute lifelessness
Non-green fluid for the clones (either red for the nice contrast, or we make as much as possible of this tech look Kaminoan), to clash with the later green
Green mists around the throne, as it’s a nice alien colour we’ve not had in a finale for a while, both evil and not just sith-red, and also as a bonus hints at the Nightsister connection for those that care.