right, a few things, and yeah, opinion follows.
I personally think that camo really doesn't suit marines. 8ft tall? 4ft wide? you need some camo! A doctrine of invulnerability doesnt really seem to suit camo.
but if youre set on it, i have some comments.
your base grey doesnt seem to cover sufficiently, you want to work all your colors so you can't see the one beneath, and i can see alot of black under the grey. The camo doesn't look that camo-y to me, and a bit thickly laid on. Also, why are the marines in urban and bolters in rural camo?
as for the sparta marine, fun idea, but watch those mould lines.