iMessages appear green instead of blue primarily because the message is being sent as a regular SMS text instead of via Apple's iMessage service. This can happen for several reasons:

• The recipient is not using an Apple device, so the message cannot use iMessage and defaults to SMS, which shows as green.
• iMessage is turned off on the sender's or recipient's device.
• The sender's iPhone is not connected to the internet via Wi-Fi or cellular data, so iMessage cannot send messages and falls back to SMS.
• There may be temporary issues or errors with the iMessage service.
• Group messages with even one non-Apple user will send messages as SMS, appearing green.
• Changes in device settings, new iPhone setup, or carrier issues can also cause this behavior.

In summary, green bubbles indicate the message is sent through the carrier's SMS service, while blue bubbles mean the message is sent through Apple's internet-based iMessage system.