Dim B As Range
Dim A As Range
Dim Cond1 As String
Set B = Cells(6, 7)
Set A = Cells(6, 8)
Cond1 = "=OU(ESTVIDE(" & B.Address & ");ESTVIDE(" & A.Address & ");" & B.Address & "<" & A.Address & ")"
With Cells(3, 3)
.FormatConditions.Delete
.FormatConditions.Add Type:=xlExpression, Formula1:=Cond1
With .FormatConditions(1)
.Interior.Color = RGB(255, 0, 0) 'Rouge
End With
End With