I had thought of that, but I'm not sure it's enough.
It works to a certain extent, but I'm not sure it would work for larger depths or larger surface areas
I don't really understand why that isn't already perfect, at a glance I can tell the depth.
If it gets darker+darker as it gets deeper, eventually it will be black, most people understand that black means it's so deep that no light gets down there.
I'd go with this too. Make the darkness based on the current elevation of the player. If the player goes down stairs, deeper tiles will get illuminated as he goes down until he passes them, after which they get progressively brighter. It will be easy to figure out how high are things no matter your current elevation.