Why do some of my messages sent to a person send as green and some as blue?

I'm texting a person who has an old iphone and they only get messages when they're connected to Wi-Fi.Sometimes my messages to them will send as green and others will be sent as blue, and some of them will look blue at first but then switches to green when it finishes delivering.
are all of them sending? The person will usually reply but it takes them a couple of hours due to the phone issue. My concern is that something won't send and I won't know it, so I'll be waiting for a response
the last message I sent was blue and it says delivered, does that mean it went through for sure?

Added (1). okay, I get that green is SMS and blue is imessage, but my question is do you think all of them are going through eventually?

Blue is an iMessage message to another iPhone user.

Green is a standard text message.

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