Why does the text message sent by Apple's mobile phone show a blue background?

Iphone sends text messages. If it is a blue background, do you think there is information on it? There will be news from iphone to iphone, and your friend must be iPhone. This is a traffic or wireless network. It costs nothing. If it is green. It's in text format. And the corresponding is the ordinary mobile phone. When your friend's iphone can't accept it, your message will fail to be sent. Click Send as Text again. It doesn't matter.