Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim ref As Range
Cells.FormatConditions.Delete
On Error Resume Next
Set ref = Intersect(Target.Areas(1), Target.Areas(2))
If ref.Count > 1 Then Exit Sub
ref.FormatConditions.Add Type:=xlExpression, Formula1:="VRAI"
ref.FormatConditions(1).Interior.ColorIndex = 3 'rouge
End Sub