No need to thange templates, just set some properties.
MetroWindow has properties:
<controls:MetroWindow ...
Topmost="True"
WindowState="Maximized"
ShowTitleBar="False" // Hide colored tile bar, title header and icon
ShowCloseButton="False"
ShowMaxRestoreButton="False"
ShowMinButton="False"
ShowWindowCommandsOnTop="False" // No window commands on top of flyouts
which will remove title bar (icon+color), and you can remove buttons too.