I don't think there's an official guideline on Land Raiders, but if you're using them for Deathwing then bone is reasonable, but since they can just as easily teleport in you could justify green if that's your preference as it stands to reason they might just use whatever land raider is available instead. I guess it just depends what you want to convey (it's their own personal ride, or they commandeered it for the mission). Green of course means you can use it with anything in your army though, whereas white would seem weirder delivering green wing units.
I have three land raiders and painted one bone white, one green and one black, to cover all the bases
Unfortunately it was a while ago so they're pretty terrible, so I'm still deciding how I plan to repaint them. One is a Crusader which perhaps makes it the best candidate for Deathwing, but is naturally the one I painted black during my youth-addled insanity