In Transport Tycoon, light comes from the right (and from above). Which means every plane faced to the right has a somewhat lighter color than the ones faced to the left. Because of this lighting effect, you can't just mirror sprites which are in opposite directions, unfortunately.
The image above shows the simple light effect. But as light comes from the sun, and the sun is in the sky, light also comes from above. With a somewhat more difficult shape this results in more different brightnesses. The image below shows such a difficult shape.