The lower speed envelope means there’s enough time for Drive Pilot to warn the human behind the wheel that it’s time to think about driving and take over. Mercedes says it picked turquoise for the marker lights in the headlight and taillight clusters as well as the side mirror because the color is differentiated enough from other colored lights that road users might encounter while driving; and human factor research points to turquoise as the optimal color for this application. California granted Mercedes approval to deploy the turquoise lights in its own fleet of test cars, initially for the next two years.

Source: Turquoise taillights tell you this Mercedes is driving autonomously