---

        private void Color_Checked(object sender, EventArgs e)
        {
            RadioButton rb = sender as RadioButton;
            if (rb == null || !rb.IsChecked != true) return;

            Dictionary<string, System.Windows.Media.Brush> colors =
        new Dictionary<string, System.Windows.Media.Brush>
    {
        {"Красный",    System.Windows.Media.Brushes.Red},
        {"Синий",      System.Windows.Media.Brushes.Blue},
        {"Зелёный",    System.Windows.Media.Brushes.Green},
        {"Жёлтый",     System.Windows.Media.Brushes.Yellow},
        {"Оранжевый",  System.Windows.Media.Brushes.Orange},
        {"Фиолетовый", System.Windows.Media.Brushes.Purple},
        {"Розовый",    System.Windows.Media.Brushes.Pink},
        {"Чёрный",     System.Windows.Media.Brushes.Black},
        {"Серый",      System.Windows.Media.Brushes.Gray},
        {"Коричневый", System.Windows.Media.Brushes.Brown}
    };

            if (colors.ContainsKey(rb.Content.ToString()))
                tbSample.Foreground = colors[rb.Content.ToString()];
        }
    }
}

<StackPanel>
    <TextBlock Text="Выберите цвет:" x:Name="tbSample" FontSize="14" Margin="0,20,0,15"></TextBlock>
    <RadioButton Content="Красный"    GroupName="Color" Click="Color_Checked"/>
    <RadioButton Content="Синий"      GroupName="Color" Click="Color_Checked"/>
    <RadioButton Content="Зелёный"    GroupName="Color" Click="Color_Checked"/>
    <RadioButton Content="Жёлтый"     GroupName="Color" Click="Color_Checked"/>
    <RadioButton Content="Оранжевый"  GroupName="Color" Click="Color_Checked"/>
    <RadioButton Content="Фиолетовый" GroupName="Color" Click="Color_Checked"/>
    <RadioButton Content="Розовый"    GroupName="Color" Click="Color_Checked"/>
    <RadioButton Content="Чёрный"     GroupName="Color" Click="Color_Checked"/>
    <RadioButton Content="Серый"      GroupName="Color" Click="Color_Checked"/>
    <RadioButton Content="Коричневый" GroupName="Color" Click="Color_Checked"/>
</StackPanel>