I'm glad you liked the art style! As for the monsters, the idea is that they are hiding in specific locations to mislead you even if you memorize the drawing. This is largely the reason why the art style is simplistic and the drawings are in black and white only; since you can only see lines and nothing is filled in, it's a matter of observation and trial and error using the glowing paint. So yes, the core game loop is essentially a memory game with "on the spot" thinking thrown in.
The game has a fairness system in place when it comes to painting required areas: if a monster overlaps any required paint areas, said areas become optional. Otherwise it would be unfair if you were required to paint some of the monster to advance. Though, I will admit the monster "detection" areas are far from perfect and could have used a bit more tuning. Also, to make painting less tedious, the "background" part of the drawings are not required, only the objects and scenery are required. However, looking back, I think there should be more information given to the player in regards to that because for some levels it can be hard to tell what is required and what is the background.