The purple loctite is made specifically for small diameter threaded bolts and fasteners as are used on sights and scopes which you want to hold tight without loosening but you want to be able to remove at a later time.
Many here use the blue which is made to be used on larger automotive style bolts like 1/4", 3/8", and 1/2" which you want to hold tight and not vibrate loose but you do want to be able to possibly remove at some time in the future.
The blue is used because it is more readily available and it works (but so does clear nail polish which is MUCH cheaper) but it is not the proper strength.
If you can get any of the three options that you want (nail polish, blue, or purple), I would suggest purple loctite or clear nail polish....<><.... :money: :grin: