Colouring cells depending on letter value

Sub colour()
ActiveSheet.Unprotect
For Each cl In Range(Cells(5, 1), Cells(576, 184))
    Select Case cl.Value
        Case ""
        Case "E"
            cl.Interior.ColorIndex = 6
        Case "OE"
            cl.Interior.ColorIndex = 4
        Case "ON"
            cl.Interior.ColorIndex = 33
    End Select
Next
End Sub

Kind regards
Leo