Private Sub Worksheet_Activate()
Dim c As Range, w As Worksheet, coul&
On Error Resume Next 'si une feuille n'existe pas
For Each c In [D31:D35] 'à adapter
Set w = Nothing: Set w = Sheets(CStr(c))
c(1, 2) = IIf(w Is Nothing, "NOK", "OK")
If w Is Nothing Then coul = 255 Else coul = w.Cells(1).Interior.Color
c(1, -2).Interior.Color = coul
Next
End Sub