Skinning: Using a Color as StaticResource for another Color

This?

<Color x:Key="DarkBrown">#C4AF8D</Color>

<DynamicResource x:Key="TextBoxBackgroundColor" ResourceKey="DarkBrown"/>
<DynamicResource x:Key="ToolBarButtonForegroundColor" ResourceKey="DarkBrown"/>

For more advanced use cases and multiple levels of aliasing see this answer.

Why don't you just make Brushes.xaml skin-specific? Then you will have this:

<Color x:Key="DarkBrown">#C4AF8D</Color>

<SolidColorBrush x:Key="TextBoxBackgroundBrush" Color="{StaticResource DarkBrown}" />
<SolidColorBrush x:Key="ToolBarButtonForegroundBrush" Color="{StaticResource DarkBrown}" />

Another point in favor of making brushes skin-specific is that there are situations where you would want to make the ToolBarButtonForegroundBrush a solid color brush in one skin and a gradient brush in another skin.