Thanks for your input.
@ratchet/ all: I would love to create such amazing art assets but it's simply not possible (for me), I'm the coder, level designer, modeler (and animator), texture painter and play tester of my game. If I had more time, I myself would go with a colorful game. It's not (just) an art decision.
@achaziel regarding Superku World: I think it's pretty difficult to handle a bright and colorful scheme, esp. if you're not the best artist. Btw. the game was never meant to a bright platformer like Mario, more like the old Metroid side-scrollers.


EDIT:
First version of new effect/ gameplay element: hidden areas: