Joke Collection Website - Blessing messages - Why does Apple's mobile phone sometimes send text messages in green, sometimes in blue, and blue messages can't be sent out?

Why does Apple's mobile phone sometimes send text messages in green, sometimes in blue, and blue messages can't be sent out?

Green is operator information, which can be sent to every mobile phone and operator; Blue is Apple's own iMessage network message, provided that both parties are Apple mobile phones, and both parties need to turn on iMessage before sending it to the other party. If the other party is an Apple mobile phone, but iMessage is not turned on, it cannot be sent, but you can choose to resend it and use the operator to charge for the message.