Why is the message received in black on a white background and the message sent in black on a blue background?

The message is received in black on a white background and sent in black on a blue background, indicating that the other party's mobile phone is not an iPhone. When you receive a message, it displays black characters on a white background, indicating that it was sent in the presence of a network. By default, it is sent by Apple's iMessage, and the SMS fee is not deducted. When there is no network and the other party's mobile phone is not iPhone, send it in black and white on a blue background.