Can be activated by engaging the autopilot system.
In order to find the Easter egg, drivers must engage the autopilot system by pulling the cruise control stalk towards them. Once autopilot is activated and a blue icon is illuminated, drivers need to pull back on the cruise control stalk four times rapidly. When this occurs, the typically grey road representation in the dashboard turns into a psychedelic cowbell road that is reminiscent of the "Rainbow Road" map in Mario Kart 64.
While this particular Easter egg isn't too exciting, previous gags have included turning the Model S in to the Lotus submarine from The Spy Who Loved Me and showing a wrap speed montage on models in Ludicrous speed mode.
Speaking of the Model S, Tesla just unveiled the facelifted variant yesterday. It features a modestly revised exterior with a new front fascia and standard LED headlights. The Model S also gains several new interior options including heated / ventilated front seats, a medical-grade HEPA air filter, and Dark Ash or Figured Ash wood trim.