What to do with the extra card in Chicago Opener?

It's simple: just pocket it, or give it to the spectator. If, by some remote chance, someone asks why you put it away, tell them it's because it has a different colored back, and you need all the cards to have matching backs, then give them a look that says, "Are you kidding me?" A well-behaved spectator won't say a peep.

@Jaume:

If I set it aside, they might say I'm now missing a card, or the card might reappear very visibly in the next trick, even though I supposedly removed it.

How many times has this actually happened to you with a real audience? We magicians often think people will suspect things that only we would ever dream up. I'd recommend that before you fret over this, you actually keep a tally of how many times someone asked you why you pocketed that odd card after the trick.

@Jaume:

A visible, suspicious deck switch

More than suspicious, I'd argue it's unnecessary.

Bottom line: when you find yourself thinking a spectator might suspect this or that, take a step back and consider if a layperson would actually suspect all those things you've cooked up. Think about what the spectator sees, but with a spectator's mindset—not a magician's, or even a spectator who knows some magic. That approach will help you shed a ton of unnecessary worries.

Cheers,

This is an effect I used to do a lot years ago. Honestly, I think it's an interesting premise.

Now, I tend to overthink these things more. Why does the card turn red? I always did it with the Ace of Spades and the Queen of Hearts. I used to say the cards were shy, which is why they'd turn red, and the Queen was the shyest of all, not only turning red but then vanishing.

Back then, the effect would just end there, and I'd move on.

Now it's harder for me; I've set up a fictional premise, and then poof! it just vanishes??

If I had to do it again now, I'd probably try to "calm" the Queen down, so she'd turn blue again, and then give the deck to a couple of spectators to shuffle, helping her "regain confidence" and "stop the cards from changing color all willy-nilly."

These are things that didn't bother me before, but now they give me quite a bit of OCD, lol. I talked about this with Luis Olmedo back in the day, regarding a version of the Magic 9 that he taught me, which I modified. Besides internal workings, I changed the external presentation so the card making the change wasn't from the same deck, but from a different one.

For those unfamiliar with it, I'll explain the effect, though you can also look it up on YouTube. It's an effect by Father Hamman, later adapted by Ascanio (it's also in the GEC). A red Nine is taken from the deck, along with X black cards. The story is that the Nine is magical and can change the cards. The black cards are brought closer to the Nine, and they turn red. Of course, for me there's a fictional problem (I prefer 'phenomenological'); for starters, how can there be black cards in the deck if the red Nine is already there? Or if I then spread the deck, will the cards next to the Nine always be red? In my case, everything revolved around introducing a card from a different deck. Once that card was removed, everything returned to normal, and I could continue.

I'd switch to a coin trick, a rope trick, or something else, or an effect where you have to introduce a different deck, like the Invisible Deck. Then, when you bring out the "original" deck again, you've already swapped it out for an identical one.