I had recently listened to Deathwing on a Youtube channel called Grimdark narrator; unfortunately GW got wind of him making narrations of their books and he's had to stop. I do remember, however, very clearly, that it is a direct result of the events told in that story that they start painting their armour bone-white. If memory serves correctly, I think it was to honour the tribespeople that they originated from, who had been subjugated by a Genestealer cult. I think the colour was representative of those who were 'living ghosts' and didn't expect to live through the upcoming battle.