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The following support article shows more about what the different colors mean in Messages, as well as why a message would send as SMS rather than iMessage.
The type of message you are sending is noted by the color of the send button and text bubble.
- A text message sent as SMS or MMS will show the word "Send" in green when typing and have a green text bubble.
- A text message sent as iMessage will show the word "Send" in blue when typing and have a blue text bubble.
Some carriers may automatically convert long SMS messages (ones that would require multiple SMS messages to send) into MMS format.
On iPhone, if iMessage is unavailable, the message may be sent as SMS or MMS, depending on the content. Carrier messaging rates may apply. You can adjust this behavior on your iPhone at Settings > Messages > Send As SMS.