@Maxw7: If the message bubbles are green, then they have been sent in SMS or MMS format, while blue bubbles have been sent as a chat format. If you open the Messages app > Tap the 3 dots in the top right > Settings > Chat settings > Default message type, and set this to Automatic, then this will adjust the format accordingly, depending on the recipient, and your current connection status. As pointed out by @Fiona26, a Wi-Fi connection is required in order to send messages as a chat, but you can also tap on 'If a chat isn't delivered' and select 'Ask before resending' if you'd prefer to maintain some format continuity with a certain contact.
