To make your conversations more alive, focused and colorful you're able to use the message theme Bubbles. This feature makes it possible to use different colors in message bubbles depending on your own messages or the messages received from a visitor or a colleague. 

To activate this feature, go to Settings, which you find by clicking on the profile picture in the lower left corner in the client. Go to Messages > Message theme and check the box named Bubbles.

To choose color, just tap the little colored box in front of My bubble color, Others bubble color and Visitors bubble color.