painting white and inking them with green/guiliman blue glaze etc then drybrushing white over (while glaze wet) will give you a nice blend/ghost look if you ask me.
you could use green stuff and make "wispy" looking spirit tendril things that meld together with their bodes/connects to bases (like GW mortis engine for VC) ... and just connect those wisps to different spots on marines.. legs melding out of the spirit on some, at the waist on others, chest on others even, could just have balls of energy for some with arms/screaming heads poking out?
just my 2c
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