During prototyping I was so focused on the mixing and the small things like how a swatch gets dragged smoothly, I finalized the "game" aspect of it in the last hour or 2. I was pretty surprised by how hard the game is myself haha.
Judging from the colours, most the time if 2 parts are the same that means you can rule out those colours, in 9F5F5F that would mean green & blue. This is complicated as if you mix green and blue that may have valid combinations but still, another thing to keep in mind is due to how merging works, blue plus red then plus white is not the same thing as blue plus white then red. Order of merging matters.
Now my solver got 9F5F5F in 3 moves but that doesn't mean you have to do 3 moves, you can do as many as you'd like as long as you get 9F5F5F. I don't know how the solver got it but I managed to find a solution to 9F5F5F in 5 merges, that being red+white+white then red+black+black and then add those together. Another solution I found is 4 merges, black+red then black+white+white then merge those. I'd be impressed if some legend could write the solution in 3 moves.