On the iPhone mobile device, messages appear green when sent via SMS, messages turn blue when sent using iMessage over an internet connection to another Apple device. iMessages aren't text messages in the traditional sense and are best described as messages sent back and forth in an application. A way of thinking about it is this:

The beauty of Medallia Concierge is that your customers are not required to download an app to message with you in. Because this is accomplished using SMS, messages sent from iPhone devices turn green.