Why is Apple's mobile phone blue when sending messages to others?

Apple phones have iMessage function (Settings-Messages-Turn on iMessage). If both parties turn on the iMessage function at the same time, the message will be sent through the network without charging the communication fee of the operator, so the font will be displayed in blue after sending. If the other party doesn't turn on the iMessage function or it's not an Apple phone, the iMessage message can't be sent, so the font is green.