I don't know if it was only on my hand but the speed of the cristal was too fast to be somewhat controlable without shouting.
On the other hand mechanics are super interesting but could be introduce a little bit better in 3 phases :
- first few level focus on subtracting color from ennemies (the right color, then a combinaison) with a timer that reset the ennemie to negate mistake
- then the new few focus only on adding color on the same principle a above with the addition of the changing cristal color mechanic
- and finally a combination of all of the above