Messages sent from Apple's Messages app are either free iMessages or traditional SMS text messages. The messages appear blue when the recipient has an Apple device capable of receiving an iMessage and green when the recipient doesn't use an Apple device. IMessages appear blue even if the recipient's iPhone is dead or turned off.
When you send an iMessage, "Delivered" appears under the message when it is received by the recipient's device. The message is stored on Apple's servers if the recipient's iPhone is turned off or dead. When the receiving iPhone is turned back on and connected to a Wi-Fi or data network, the recipient can read the iMessage.
Identify iMessage or SMS Messages
SMS messages and iMessages are distinguishable by color. While there is no charge for iMessages exchanged between Apple devices, an SMS message may incur a charge from the recipient's cellular provider, depending on his plan.
IMessages are always blue. They are sent to recipients who use a Mac computer or Apple mobile device equipped with the Messages app.
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SMS messages are always green. These are traditional text messages.