How to solve the problem of sending blue SMS by Apple 4S?

Blue indicates that the information sent out is sent through iMessage, which does not generate SMS charges and requires traffic. In iMessage mode, if the message fails to be sent, it will be automatically sent by SMS.

Green means sending by SMS, and the information sent out is charged according to the operator's tariff.

I hope it's all blue. I don't know how many times the traffic is cheaper than SMS.

If you want to send by SMS, you can turn off iMessage in the settings.