Hey there Dark-Aii! Just played your game, all the levels. Great to see such a nice game made on Godot. Also, maybe you can get more feedback on /r/MadeWithGodot. Just a idea!
Ok, now to my feedback. I used the Windows version; the HTML5 seems to not work (on the first stage, you can't control the ball and it doesn't move).
The gameplay was interesting: different balls having different properties was nice. To be sincere, playing was kinda boring at the beginning, because it was very straightfoward. However, when the rotation mechanic was introduced, then I got a lot more invested on the game! Before I continue, I would like to say that maybe the game needs to be a bit boring at first, so that the new mechanics/colors can be completely understood by the player before introducing a new one: then, I believe the pacing of the game right now is good (even if I did not like it at first).
On possible improvements, first, a (major) bug I found: when entering a level or resetting it, sometimes the change rotation button (R) don't work. Such thing makes playing throught the game kinda stressful, changing the focus from puzzle to patience. Hopefully it is easy to fix. Also, as I personal opinion, I didn't like the effect of the black balls on the transparency. However, this is just my opinion! Maybe you should leave such effect in the game. Another opinion mine would be to change the font on the menu that appears when you finish/fails the level: it is a bit hard to read.
On Design Advice, I think I cannot help a lot other than say that the whole spectra of colors can be turned into different mechanics! (laughs) But seriously, there's plenty of room for more interesting mechanics other than change color and change mass and change number of connected balls. Maybe you could add a pair of balls with magnetism effects (attraction and repulsion); balls that only appear when you are of a certain color; a color that can absorb any other (including itself); or maybe a color that inverses the rotation (in this case the Change Rotation button could be removed). These are just some quick ideas, I'm sure someone else or yourself can think of a lot more other interesting stuff.
On how I felt playing the game, at first I was a bit bored, then I got interested, then a bit nervous/stressed (some sections needed a lot of precision and my change rotation button wasn't working... so, not fault of the game design anyway), and then interested again by the last level. I guess that if the controls were working correctly, the experience would be pretty nice overall. Also, the music fitted nicely with the game, and surely contributed to its positive feel.
On helping you, I guess the best that I can do is provide this feedback, along with ideas that I said. If you think I could help more, just send a email (you can find a email to contact me on my profile). Good luck on making this game better and publishing it!