Randomly sending green messages for a few moments

Hopefully the following will help clear things up.

• iMessages (blue bubbles) require an Internet connection. That connection can be either via Wi-Fi or cellular data. iMessage is a service provided by Apple. iMessages can only work between Apple devices.
• SMS text messages (green bubbles) rely on a cellular voice connection. SMS is service provided by your cellular provider. SMS messages do not require an Apple device and is what an Apple device uses to send/receive messages with a non-Apple device (think Android).
• Cellular voice & cellular data are two different services that your cellular provider offer. Neither are provided by Apple.
• The Messages app on your iPhone supports both iMessages and SMS text messages.

One reason when sending/receiving iMessages that they "turn into" SMS messages is do to a loss of Internet connectivity. This often happens when your iPhone is not connected to Wi-Fi.

I already provided you with one reason. If your phone loses Internet connectivity, even momentarily, the Messages app will make one "last ditch" attempt to send the message. Since neither Wi-Fi or cellular data is available, it resorts to using cellular voice (green bubble). This typically happens when you are in a remote area with limited cellular coverage, but could happen at any time.

No, not being "hard headed" at all. You are asking very reasonable questions. Unfortunately, technology does not "always work," and borders on a bit of magic and luck. I really don't have an answer to why this happens. There are too many unknowns to provide you with anything with any certainty. At best, from what you provided, I would suspect the stability of the Internet connection where you are using this phone at maybe the cause ... but this is only a guess. Without performing some network diagnostics, it would be impossible to know for sure.

There are a lot of very knowledgeable folks here that probably could provide you with a better answer. Hopefully, someone will chime in to offer it to you.

... again, that's because iMessages still work with either Wi-Fi or cellular data to access the Internet.