Option Explicit
Private Sub Workbook_Open()
Dim SAYFA As Worksheet, AY As String
AY = UCase(Replace(Replace(Format(Date, "mmmm"), "ı", "I"), "i", "İ"))
For Each SAYFA In Worksheets
If UCase(Replace(Replace(SAYFA.Name, "ı", "I"), "i", "İ")) <> AY Then
SAYFA.Tab.ColorIndex = xlColorIndexNone
Else
SAYFA.Tab.ColorIndex = 3
End If
Next
End Sub