(nouvelle pièce jointe, si j'ai bien compris
Sub test_nouveau()
Dim y As Double
Dim z As Double
For n = 3 To Range("C65536").End(xlUp).Row
If Range("C" & n) <> "" Then
If Range("C" & n).NumberFormat = "000""/""####""/""#####" Then
Range("C" & n).Interior.ColorIndex = 4
Else
Range("C" & n).Interior.ColorIndex = 3
End If
End If
Next n
End Sub