Could the colour have been bleached out by sunlight?
Good thinking, that's another possibility.
Simple way to tell would be if any part has different intensity on different faces.
Edit: In fact, looking at the pictures I think you are right, because the pattern of colours on opposite faces don't match up, indicating that each part has different shades on opposite faces.
* Unless each piece is made by gluing two halves from different batches, but I doubt it.