Am not sure what material design guidelines include, can somebody summarize them in relation to the kontalk icon?
Who designed this @daniele_athome? It would be nice to hear from the designer(s) themselves explaining their thinking before deciding.
I think that ideal shade of colors for me would fall somewhere between the very bright of the old icon and the darkness of the new. And am not sure about that black line on the new one, even though I don’t like the thickness of the white lines on the first one. 
We need to mix them into a new icon! 
As you can see I haven’t decided yet. Too many fine details in the way!