Not sure I understand how the user will input the required format but be aware you current code will not work if the user selects characters in the word - Only those selected will change. If you need the WORD to change regardless of the selection you wil need more complex code.
Sub RedStrikethrough()
Dim otr2 As TextRange2
Dim allText As TextRange2
Dim L As Long
If ActiveWindow.Selection.Type = ppSelectionText Then
Set allText = ActiveWindow.Selection.ShapeRange(1).TextFrame2.TextRange
Set otr2 = ActiveWindow.Selection.TextRange2
L = selectedWord(otr2, allText)
allText.Words(L).Font.Fill.ForeColor.RGB = vbRed
End If
End Sub
Function selectedWord(otr2 As TextRange2, allText As TextRange2) As Long
Dim L As Long
For L = 1 To allText.Words.Count
If otr2.Start < allText.Words(L).Start + allText.Words(L).Length Then
selectedWord = L
Exit For
End If
Next
End Function
This might need to be even more complex to NEVER fail. It is not clear if the user can select multiple words.