When I finally get back into GMing I'm gonna be more upfront about it, but I'll still have a mini "password" thing set in place. Maybe ask players questions about events that transpired before the RP starts, so they'd either have to go back and read over the OP for the answers, or they'll have already read the OP and can tell me the answers at that moment. Either way, they read the OP, so it's a win-win. It would really all depend on what information I put in the OP. I still think that having a password the same hex code as the background is a funny little idea, if not a little bullshitty. But placing the password in plain sight, and even bolding it, is dumb. But roleplay passwords in general are sort of dumb, since it gets a bit obvious that there's a password when everybody mentions their love for 50 Shades of Grey in their character sheets.

In the end, though, it's just a way of instantly filtering out people who just read certain parts of the OP, or don't read it at all.