As a colorblind user I'm not quite sure what color the send+mic button is, but I'm drawn to it much more strongly than the normal send button. At the very least you'd think they would have had a pop-up confirmation message...

As a non-colorblind user, me too. The orange is a lot more vibrant and attractive than the blue and doesn't convey much of a "warning, something different" message.

It doesn't seem like Google tested this on many real-world users before deploying...?

This exactly. My eye was drawn to the orange button last night as I went to send an email to my professor. I nearly clicked it, not remembering the date. Luckily, I just thought it might be some new weird voice feature I must not understand...that was close.

As a person who reads and is mindful of what I click, I click the button that says what I want it to do.

Except they both say send. The stupid icon on the other one looks like a maraca. I would have never guessed "mic drop." Let's ignore that "mic drop" is a cultural expression that someone who grew up in a different country may not be familiar with. To be honest it look like a "send and play music" button to me.

Not just you. I've lived in the USA since I was a young lad and work for Google. I had never heard of 'mic drop' until this prank.

Then again, I don't watch TV so maybe I missed out on it being used.

I grew up in the Northeast US. I only heard the expression be used in the last year or so. I'm not even sure what it means. I do watch TV.

Someday when you accidentally click the wrong button in something, perhaps when you are old or suffer from some disability, I hope that you remember this comment of yours.

Furthermore, the icon doesn't have anything for you to read, since it uses an image (whose meaning may still not be clear to an older or disabled person; it kind of looks like an ice cream cone to me).

As a person that sends dozens, sometimes hundreds of emails a day, I click the button that's I always click.

A UI that requires the user to read every button and look at every icon that they click on because it may have been changed to do something completely different is a terrible UI.

While I might notice a new color, I'm not certain that I would.

I just don't actually believe you. I haven't observed a _single user ever_ who consistently maintains the behavior you describe across all interactions they have.

In doing usability design even for astronauts or pilots I don't think you could fully trust a user to behave as you describe.

Regular archived conversations go back to inbox everytime there's a new message.

Under the hood perhaps is it muting [1] the conversation in GMail?

  If you're part of a long message conversation
  that isn't relevant, you can mute the conversation
  to keep all future additions out of your inbox.

[1] https://support.google.com/mail/answer/47787?hl=en