Thank you all, I'm glad you like it.
Yes, water/ lava is a vertex & pixel shader. There's still a fullscreen tint active but I will probably only change it for bossfights, lostclimates's idea is nice, though. There may be a level where you have to adjust the tint (with switches) to pass certain areas, let's see how that will work out.

EDIT: The detail layer for the walkable area sounds nice, however all levels are already designed in a way that you can easily distinguish the 3 layers foreground I (walkable), foreground II (unreachable, 1/2 color saturation) and background.