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Typically a graphical style where there are one two colors. (A bit can be either on or off, so two options) Commonly black and white, but can be any combination depending on the style the creator is trying to accomplish. 

A metroidvania is a game inspired by Metroid and Castlevania

It often involves exploration and gaining new abilities to access new areas or progress.

Actual NES games or games inspired by the restrictions of an NES.