Windows 10 supposedly gives developers the freedom to change their own titlebar colors. I have found a few resources on how to do this for Windows Store apps (here, here).
But what is the underlying Windows API function for it? How can this effect be achieved in native C++?
It is now done with DWM : Custom Window Frame Using DWM
(tested on Windows 10 in C++, C# and VB.NET by drawing gradient caption bars)