It's done because I've got two other primed but unpainted tyranids ready to go and that feels like more fun than working on this one any more, which is good enough for the table. This is the way.
It's just drybrushing! If you look at the original model the detail on the armour is fantastic so it was really easy to drybrush some blue on. I did use quite a small drybrush to keep it accurate. I think on the actual GW nids those panels are flat so you need to actually draw the lines on to mimic this texture.