' Use a Red pen for the underline text decoration.
Private Sub SetRedUnderline()
    ' Create an underline text decoration. Default is underline.
    Dim myUnderline As 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.
    Dim myCollection As New TextDecorationCollection()
    myCollection.Add(myUnderline)
    TextBlock2.TextDecorations = myCollection
End Sub

' Use a Maroon pen for the baseline text decoration.
Private Sub SetMaroonBaseline()
    ' Create an baseline text decoration 2 units lower than the default.
    Dim myBaseline As 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.
    Dim myCollection As New TextDecorationCollection()
    myCollection.Add(myBaseline)
    TextBlock2.TextDecorations = myCollection
End Sub