This background color of the application icon in Windows 8 is the default behavior of Windows 8. It displays the background color of the application icon using the "dominant" color of the application icon. If your icon has a dominant brown color, it will display a brown background.
This is also the same default behavior when you hover over the launch of application icons on the Windows 7 taskbar. To improve the look, try changing the application icon to use neutral colors or light colors instead of dark colors.
Or, you can change the icon of your application to use a single colored icon (for example, white) on a transparent background to reflect flat Windows Store application theme icons in Windows 8 or use full rectangle icons such as games in Windows 8.
Example:
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