The green line is the predicted line calculated from the start of your trip.
The grey line is predicted line calculated live during your trip at any time. So if you go downhill, the grey line would change to as if you have more energy while the blue line is static because it did its job at the start of the trip and not about to recalculate in the middle.
I am not sure whether the grey line is dependent on your consumption tab where you want it to calculate instantly or at an average of 5 miles, 15 miles, 30 miles...
Trip: If your Model S is equipped with navigation, you can monitor the amount of energy being used while navigating to a destination. You can track actual usage against the initial prediction. The green line represents the actual usage whereas the gray line represents predicted usage. To change the zoom level, touch the zoom icon located in the top right corner of the chart.