Perhaps 
Sub Rufles()
Dim l As Long

For l = 3 To Range("L" & Rows.count).End(xlUp).Row
    If IsEmpty(Cells(l, 12)) = False Then
        If IsNumeric(Cells(l, 12)) And Cells(l, 12) <> Cells(l, 11) Then
            MsgBox Cells(l, 12).Address(False, False) & " is Red!"
        End If
    End If
Next l
End Sub