I am currently developing this concept. Keep in mind that this never happaned in the book or the game, and I made up the entire creature. I am just using the post apocalyptic Metro concept to bring some explanations and reasonings for what is happening.
In this concept a giant radiated bug breaks through the barricade on the surface of the metro (probably due to malfunctioning steel gates), and people down in the metro are alerted by this noise. They put on their gasmasks and rush to the escalators to see what is happening.
First up my grey tones withoout the colour layers applied:
Colour layers on:
Bug colour layer set to overlay (is this better then regular colour?):
GreyScale of the regular colour image (does it look better then normal grey scale?):
Detail of the human: