P8:AM90!? Thats a lot of cells that could have formulas. Anyhoo..

Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("P8:AM90")) Is Nothing Then
If Not Target.Cells.HasFormula Then
Target.Cells.Font.ColorIndex = 3
Target.Cells.Font.Bold = True
End If
End If
End Sub

Does this do what you want?
If you want to remove the red bold when a formula is re-entered:

Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("P8:AM90")) Is Nothing Then
If Not Target.Cells.HasFormula Then
Target.Cells.Font.ColorIndex = 3
Target.Cells.Font.Bold = True
End If
If Target.Cells.HasFormula Then
Target.Cells.Font.ColorIndex = 1
Target.Cells.Font.Bold = False
End If
End If
End Sub

Regards - Dave.