Sure, it's reasonable to talk that way, but you're not really seeing the absence of green paint on the wall. You're seeing the white paint that makes the patch white instead of green. And sure, you can "see" a shadow, but it's not really seeing anything except the light that defines the shadow by its absence. I think, in terms of the darkness spell, it's pretty meaningless to say that seeing a shadow is seeing "something" in the area of the spell, which shouldn't be allowed because you can't see things in a heavily obscured area, because then you'd have to say that someone looking into the spell area couldn't even see the darkness inside of it.
This is mostly a matter of aesthetic preference to me, but I dislike the idea of there being a mist or any other medium by which the sphere is darkened. My preference is that it's filled with a kind of elemental darkness, so completely transparent like the air of a pitch-dark room.
Rather than an obscuring effect on things on the other side of the sphere from an outside observer, which would imply to me they would be hidden or unseen like things inside the area of effect, I mentioned just up-thread the idea of a lensing effect, like looking through a darkened glass globe. That would be my preference.