It’s white by default and you couldn’t change it. We posted a workaround how to get color on your Title bar back then. But not any more, because since Windows 10 build 10565 this feature is built right in the system.
To turn on the option that shows color on Start, Taskbar, Action Center, and Title Bar, go to Settings → Personalization → Colors, and slide the option Show color on Start, taskbar, action center, and title bar on. The option is located right after the accent color tiles.
What’s more, if you want the color automatically changes based on the background image, slide on the option Automatically pick an accent color from my background.
This turns out to be my favorite setting for my Windows 10 computer. With the combination of showing color on Start, Taskbar, and Title bar and picking the color automatically from the background, I am able to make my Windows 10 desktop quite vivid and interesting.