4 KudoZ points awarded for this answer. Comment: "Thanks indeed to both of you (and endorsers two!): your excellent answers came just in time and the word "tint" was crucial. I'm going with Ziad because I found some evidence of this expression on the Web and used it in the translation prefixed with "opaque". I have since also found "security tint envelope". The idea is that because of the interior tint the contents remain totally concealed until the envelope is opened. Total respect Derek"