This location can also be reached by the upper grass entrance in the Debug Room in version 0.02.5.
It appears to be a completely desolate Asian city lacking in NPCs, and its name comes from the fact that the entire area is completely devoid of any true colour besides Fluorette herself.
Monochrome Prayer can be found here.
The Monochrome City Spirit Murder occurs here.
In version 0.03, if you kill the NPC that gives you the Monochrome Prayer, an event is triggered. If Flourette is caught by the creatures that appear, she will wake up in a dark version of the real world.