Just paint the model grey or black. Keep the skin achromatic, and avoid any bright colours. None of the parts in the box are for any particular allegiance, so you can build it any way you want.
looking at the box examples
, just emulate anything but the blue and red schemes. One you paint the model any colour but Grey/Brown/Black, it'll lean towards a god's colours.
Most folks don't attempt what you're trying to do, as often their armies are themed around a specific god to better synergise with other units in the army, so I don't think you'll find too many examples, besides looking for Daemon Princes people made for 'unaligned' Chaos Marine Legions, like Word Bearers, Iron Warriors, and Black Legion. You could also use Belakor as a base for your model, or inspiration, as he's Unaligned.