I've created a new WPF app using the ModernUI WPF Application template, because I wanted to see how I could style the contents and apparence of the ModernButton. The very first thing I did was try to use the color we've settled on (kind of an off purple). It was completely ignored. OK, why was it ignored? Here's the XAML:
<mui:ModernButton Margin="0,50"
Content="This is the button"
Background="#FF68217A"
HorizontalAlignment="Center"
Foreground="Black" />