Why are some messages written in blue,others in green

The Green messages are SMS messages To and From those contacts who either do not use iPhones or do not use iMessages. The Blue messages are iMessages To and From those contacts who use iPhones AND use iMessages.

Axel F.

Ones in blue are iMessages sent/received from other iPhone/iDevice users. Ones in green are SMS texts from/to someone who does not have an iDevice.

Blue messages were sent by iMessage.

Green are SMS messages.