@eishiya Wow! Thank you so much for all advices! About color count and AA: Well I've just learned about composition rules ( I used rule of thirds on this piece) and color harmony so I just wanted to do something beyond my limits and forget about pixel art rules for a moment :) Highlights on the walls supposed to be pieces of shiny metal, and pixels under the ball were supposed to be falling sand to add some life and movement. Thanks again! Next piece will be much better! :)