If a lot no-one walks the same route, but everyone crosses a certain place, call it y, then that would turn, while the places arround it, x and z for example, would stay as green as possible, and these color differenses just dont happen in real live.
So i dont think you should use a counting system per tile.
And most people will always use the shortest route possible, so that would always be a different -color-, so if you make roads between town that is the fastest (like straight over the mountain) you wont be needing the code at all since roads do not change color