As you might have noticed if you downloaded the latest Windows 10 Build 10056, for some reason, window titles are always grey in this build. Additionally, by default, the operating system does not use any color for the taskbar and the Start menu. Under the hood of these changes, you will find a new colorization engine which is different from the one shipped with Windows 7/8/Vista.
Open the Settings app and go to Personalization\Colors.
Enable the option "Show color on taskbar and Start menu".
Note that the "Intensity" slider is also now ignored by the system. That's because the actual colors used by the taskbar, Start menu and the Notification Center are "immersive". While the "old" Aero window borders could be of any color you wanted, these changes in Windows 10 show us that Microsoft has locked down the color choices to the same predefined restricted set of colors used for the Start screen in Windows 8.1 and Windows 8. This means the end of the classic Aero appearance we know since Windows Vista where we could set any color we wanted.
Besides this change, Windows 10 build 10056 allows you to override the taskbar color, so you can get something like this:
So it looks like Windows 10 will be a completely different operating system yet again than the Windows versions we already know and use. Unlike Windows 8, where the classic Desktop portion of Windows and Modern UI exist separately, Windows 10 will primarily be a Modern UI-based operating system where you will be able to launch classic desktop apps and not vice versa. The end of the desktop Aero engine is one of the ominous signs of this trend.