What do blue and green represent when Apple sends messages?

Blue represents iMessage information, which is sent through Wi-Fi or cellular data network; Green represents traditional SMS/MMS, which is sent by operators.

IMessage is an instant messaging service developed by Apple. Users can use iMessage to send messages such as text, pictures and videos. When users send iMessage using Wi-Fi or cellular data network, the color of the sending box is marked as blue. When users send short messages or multimedia messages, they need to send and receive them through the SMS gateway provided by operators, so the color of the sending box is marked green.

Besides color, iMessage is different from SMS/MMS. Compared with SMS/MMS, iMessage has more functions, supports more file types and characters, is not limited by SMS duration, and can be synchronized between multiple devices.