Sub Macro1()
Dim oRng As Range
    Set oRng = ActiveDocument.Range
    With oRng.Find
        Do While .Execute(findText:=",")
            oRng.MoveStartUntil Chr(32), wdBackward
            oRng.Font.ColorIndex = wdRed
            oRng.Collapse 0
        Loop
    End With
    Set oRng = Nothing
End Sub