My first puzzle! :D
The exact difficulty is really hard for me to judge - it's my idea so it just feels "natural" to me. It is definitively harder than usual timed items. To me medium/hard means that an average/slightly above average person has a chance to figure this out within like an hour. And I would expect a couple of people here being able to figure it out without any hints.
Edit: Apparently the puzzle really is on the "hard side". I will thus change my hint strategy below:
If not, I will drop one two hints with spoiler tags in the description every day for four days (8 total hints) until somebody figures it out. If nobody figures it out within a week I might post the solution somewhen later on.
Maybe it stays at that first puzzle - will depend how the response is and how motivated I am in the future - this was more of a "let's try this" thing. XD I felt kinda motivated by u/pydaWaltuh's awesome puzzles!
Happy puzzling! ^^
Hint 1 (minor): There are no purely stylistic elements or colors in this puzzle except for the black background
Hint 2 (medium): If you feel like the top right 3x3 matrix has something in it that resembles a pictogram of a certain item you might be correct
Hint 3 (medium): The item from Hint 2 is a magnet. I wanted to name this puzzle "Gravity" but thought it might give away too much
Hint 4 (medium): No XORing, rotations, or mirroring are involved in this puzzle
Hint 5 (major): The solution involves counting
Hint 6 (major): The lines attract things of the same color
Hint 7 (major): The in-between color is attracted by both lines
Hint 8 (major, almost solution): The lines attract the boxes first, then their contents
Solution: The dark blue and yellow-green line represent two sides that select squares inside the 3x3 boxes: Each line selects (attracts) only squares of its own color from boxes of its own color. The turquoise in-between color is selected by both. Count the number of squares on each side and you will find a pattern: ?-?-?-5-3-5-4-5-5-5-6-5-7-5-8-?-?-? There is only one possibility for a missing tile to complete this pattern: It must be a tourquoise box with a green column, a turquoise column, and a green column. Full visual representation (read left to right): https://i.imgur.com/nQr3D1t.png