The UAC prompt will appear here you should allow it to write new color settings to the Windows Registry.

Pick a color you like and click the 'OK' button. This dialog will allow you to choose from a range of colors which are available for the logon screen.

your personal color settings of the Start screen. It will read and show the preferences of the current user, e.g. Here's how you go about changing the color:

To change the logon screen's color, I've coded a little tool called Start Screen Color Tuner for Windows 8.1.