I had already thought about using the character as an ink marker (with the previous white character). Fill it with red depending on the remaining ink. But I changed the main character to make it more attractive. That is why I implemented the test tube system.
Regarding the star system, I plan to put 3 coins (or something similar) in each level. Now I'm fixing some bugs in my previous game, but I'll get to work as soon as I send the update to Google Play.
Thank you very much for your advice.
PD: Sorry for my poor english :(