Hello Gianmaria, sorry for answering in english. I hope that’s ok with you.
You are asking about the three build-in colors – and how to change them.
In Alpha 3 we do not have coloring done yet. Colors and painting functionality will come in Alpha 4.
The colors you see now, is simply colorcoding to distinguish the layers from each other. You can set each of your layers to one of the build-in colors.
From Alpha 4, coming out next, you will be able to create your own colors and do fill and paint, etc.
I hope that answered your question? Otherwise let me know. 😉