Why are some of Apple's short messages blue and some green?

The green one is normal SMS, and the operator will charge SMS fee. The blue one is iMessage, which can only be used between iPhone and costs nothing. The premise is that the mobile phone turns on iMessage and the network, because iMessage uses traffic. The opening method of iMessage is as follows: (1) Select "Settings" on the mobile phone desktop, as shown below; (2) Select "Information" on the setting page; (3) You can see iMessage on the information page, turn on the switch, and log in to AppleID to use iMessage (cellular data must be turned on).