defining contrast in cnc carving

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3/ I did try spraying the whole surface black an cutting through - but this gave me a light line against dark and I'm looking for the reverse really

How about lightly spraying everything black, then skimming/sanding the surface to return the uncarved bits to the original colour? Alternatively laminate some thin light oak onto some dyed wood or dark coloured Valchromat and carve through.