Say, I have three opened notebooks Untitled-1, Untitled-2, Untitled-3 in first instance of Mathematica and three notebooks Untitled-1, Untitled-2, Untitled-3 in second instance of Mathematica. How do I distinguish which notebook belong to which instance? Is it possible to have notebooks of one instance to have different window border color then notebooks of another instance? Preferably that it would be done automatically - i.e. each new instance of Mathematica would choose different color of window.