Option Explicit

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
  'If the target cell is clear
     If Target.Interior.ColorIndex = xlNone Then

        'Then change the background to the specified color
        Target.Interior.ColorIndex = 6                          '6 is yellow

        'But if the target cell is already the specified color
        ElseIf Target.Interior.ColorIndex = 6 Then

        'Then change the background to the specified color
        'Target.Interior.ColorIndex = 3                         '3 is red

        'But if the target cell is already the specified color
        'ElseIf Target.Interior.ColorIndex = 3 Then

        'Then clear the background color
        Target.Interior.ColorIndex = xlNone

    End If
End Sub