The object of each puzzle is to make all the objects the same color by drawing lines that begin and end with the same color, with one different color in between which will change to the end color.
The visuals and calming music work well for this type of game. Puzzle complexity with different shapes come in gradually.
So why no higher than a 3? First, I cannot find any text explaining the puzzles. I guess that it's designed for exploration. Do not assume anything. At first, I thought that all lines had to start with a circle. Nope!
The "fun" designed into the game appears to be the a-ha! moments within it. The issue is that sometimes, it becomes a puzzle of trial and error. The larger the puzzle, the more errors. And the removal of the fun.