<Button Name="ToggleButton" Height="40" Width="40" Cursor="Hand" Style="{DynamicResource PlayerButton}">
            <Grid Width="10" Height="15" >
                <Path Name="ToggleButtonPath" Data="M8 5v14l11-7z" Fill="White" Stretch="Fill" Height="15" Width="11"/>
            </Grid>
        </Button>
        <Button Name="NextButton" Height="40" Width="40" Cursor="Hand" Style="{DynamicResource PlayerButton}">
            <Grid Width="10" Height="15">
                <Path Name="NextButtonPath" Data="M8 5v14l11-7z" Fill="White" Stretch="Fill" Height="15" Width="11"/>
            </Grid>
        </Button>

((Path)((Grid)Button.Content).Children[0]).Fill = ...