Sub ListChange()
Dim r As Range
Dim MyList() As String
Dim i As Long
MyList = Split("challenging,difficult,down by,", ",")
For i = 0 To UBound(MyList())
   Set r = ActiveDocument.Range
      With r.Find
         .Text = MyList(i)
         .Replacement.Highlight = wdGreen
         .Execute Replace:=wdReplaceAll
      End With
Next
End Sub

Sub ListChange()
Dim r As Range
Dim MyList() As String
Dim i As Long
    MyList = Split("challenging,difficult,down by,", ",")
    For i = 0 To UBound(MyList())
        Set r = ActiveDocument.Range
        With r.Find
            Do While .Execute(FindText:=MyList(i))
                Select Case i
                    Case Is = 0
                        r.HighlightColorIndex = wdYellow
                    Case Is = 1
                        r.HighlightColorIndex = wdTurquoise
                    Case Else
                        r.HighlightColorIndex = wdGreen
                End Select
                r.Collapse 0
            Loop
        End With
    Next
End Sub