// Use a Red pen for the underline text decoration.
private void SetRedUnderline()
{
    // Create an underline text decoration. Default is underline.
    TextDecoration myUnderline = new TextDecoration();

    // Create a solid color brush pen for the text decoration.
    myUnderline.Pen = new Pen(Brushes.Red, 1);
    myUnderline.PenThicknessUnit = TextDecorationUnit.FontRecommended;

    // Set the underline decoration to a TextDecorationCollection and add it to the text block.
    TextDecorationCollection myCollection = new TextDecorationCollection();
    myCollection.Add(myUnderline);
    TextBlock2.TextDecorations = myCollection;
}

// Use a Maroon pen for the baseline text decoration.
private void SetMaroonBaseline()
{
    // Create an baseline text decoration 2 units lower than the default.
    TextDecoration myBaseline = new TextDecoration(
            TextDecorationLocation.Baseline,
            new Pen(Brushes.Maroon, 1),
            2.0,
            TextDecorationUnit.Pixel,
            TextDecorationUnit.Pixel);

    // Set the baseline decoration to a TextDecorationCollection and add it to the text block.
    TextDecorationCollection myCollection = new TextDecorationCollection();
    myCollection.Add(myBaseline);
    TextBlock2.TextDecorations = myCollection;
}