Atmospheric perspective
Also said as Aerial perspective, Depth haze
Far things look paler, bluer and lower in contrast, because there is more air between you and them. Painters have used this for centuries to make a flat surface look deep.
It separates layers without outlines, and the play layer gets the contrast by default. Without it, a mountain and a nearby rock argue for the same attention.
Say it
“Add atmospheric perspective: each background layer shifts 25% towards the sky colour and loses contrast, so the farthest ridge is barely darker than the sky.”
Where you’ve seen it
Limbo layers its grey world so each step back is paler than the last, with the near plane almost black. It has no colour to separate its layers, so this does all the work.
Not to be confused with distance fog
Distance fog is often there to hide what the machine cannot draw, as in the fog of the original Silent Hill. Atmospheric perspective is the natural effect that fog imitates, and it is about depth, not hiding.