lib/metro/transitions/fade_transition_scene.rb
def show
      rectangle.color = starting_color

      color = final_color

      animate :rectangle, to: { red: color.red,
                                green: color.green,
                                blue: color.blue,
                                alpha: color.alpha },
                          interval: interval do

        transition_to next_scene
      end
    end
def show
      rectangle.color = starting_color

      color = final_color

      animate :rectangle, to: { red: color.red,
                                green: color.green,
                                blue: color.blue,
                                alpha: color.alpha },
                          interval: interval do

        transition_to next_scene
      end
    end
When the scene is shown set up the starting color for the rectangle
 and queue the animation to transition the color to the final color.