"Posterization of an image occurs when a region of an image with a continuous gradation of tone is replaced with several regions of fewer tones, resulting in an abrupt change from one tone to another. This creates an effect somewhat similar to that of a simple graphic poster."
This was made to prove that i can posterise Saigon in two hours tomorrow. Obviously if i were doing this seriously i'd make it less flat and fugly, but for now this is my 5-minute version of posterisation!
Original Image with city selected with magic wand
Cut out city
Play with curves, saturation, and channel mixer to be set to monochrome (original layer)
Duplicate layer and invert (inverted layer)
Color overlay the original layer below
Blend multiply the inverted layer over original layer