How does the blue-black background of Huawei mobile phone become black and white?

Set in the reading software to switch between day mode and night mode.

Why is the message received by 4s mobile phone in black and white? When I replied, it was white on a blue background, indicating that the other party's mobile phone was not an iPhone. When you reply, it is displayed in white on a blue background, indicating that it was sent when there was a network. By default, it is sent by Apple's iMessage, and the SMS fee is not deducted. If there is no network, it is sent in white on a green background, which is a normal SMS. If both parties are iPhone, the default is to use iMessage to send.