Overriding system colors will not work with custom templates.
"IsSelectionRangeEnabled" true, "SelectionStart" "SelectionEnd" . "SelectionEnd" "Value", , ...
<Style x:Key="SliderStyle" TargetType="{x:Type Slider}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Slider}">
<Grid VerticalAlignment="Center">
<Border x:Name="borderBackground" Margin="6,0" Height="4" Background="Gray" />
<Canvas Margin="0,-4,0,0" VerticalAlignment="Center">
<Border x:Name="PART_SelectionRange" HorizontalAlignment="Left" Height="4" Background="{TemplateBinding Foreground}" />
</Canvas>
<Track x:Name="PART_Track">
<Track.Thumb>
<Thumb Width="10" Height="20" />
</Track.Thumb>
</Track>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
<Setter Property="IsSelectionRangeEnabled" Value="True" />
<Setter Property="SelectionStart" Value="{Binding Minimum, RelativeSource={RelativeSource Self}}" />
<Setter Property="SelectionEnd" Value="{Binding Value, RelativeSource={RelativeSource Self}}" />
<Setter Property="Foreground" Value="Red" />
</Style>