iMessage

The Blue iMessages go through when both the sender and the receiver are on iPhone AND have iMessage activated. The Green Messages are SMS messages when the recipient is not using an iPhone. Hope this answers your question. There is nothing you can do fix that because there is nothing wrong with this behavior in Messages.

Axel F.

Just to add to what Axel Foley has said, you need to have internet to send as iMessage else it will be sent as SMS based on you settings. Settings > Messages > Send as SMS [Enable], with this settings when iMessage is not available due to no internet, the message will be sent as SMS (Green)