为我的窗户改变Aero玻璃的颜色？

I'm using DwmExtendFrameIntoClientArea in my WPF application to get the glass effect. This is working fine. What I'd like to do is change the colour used for the glass -- I'm writing a countdown timer, and I'd like the window to be the normal glass colour most of the time, and then to go red (but still with glass) when the time runs out. in my WP