At some stations I've gotten blue BPs; this has on occasion resulted
in an incredulous stare and an apology when I've signed in at the
club. "Why'd they give you a blue boarding pass?""I have no idea,
that's what the machine gave me. It's no big deal." "Yes it IS [handing
me a yellow one]."

I think the yellow ones were an expensive, stupid idea to begin with.
It would have been easy to differentiate premium passengers via the
big printed numbers, without the rigmarole of having two paper paths
and two inventory items.