Both of course are self-seal envelopes where you turn up the bottom flap, then turn down the top flat, and on contact the envelope seals. With security tint envelopes, as the name implies, the inside lining of the envelope is tinted. Security tint envelopes are designed for sending sensitive information through the mail. They are most often used for financial documents, personal information like medical records, checks, government correspondence and anything else the sender or receiver wants to protect from unwanted viewing. The security tint is usually blue, but sometimes these envelopes also come with security tint in black Simple Seal. Simple Seal w/Security Tint.