I agree with what Rachel said, but if you have to base it on an existing SolidColorBrush, you can do it with the following:
<SolidColorBrush x:Key="MyDataGridHeaderBrush"
Color="{Binding Source={StaticResource HeaderBrushDefinedElsewhere}, Path=Color}"/>
Note this just works for the “Color” attribute, you’d have to do it separately for each attribute you needed.