Your teacher is correct. Cupping is an excellent treatment modality, but also a great diagnostic tool. The level of coloring will indicate the status of the underlying tissues and to some degree the status of the persons overall energy.
For this to be accurate, however, you have to be fairly sure that the cups were applied with even pressure. By this I mean that the marks will be lighter based on how strong the cups are put on regardless of the underlying pathology. Some areas are also more prone to being darker than others due to the amount of muscle tissue below the surface.
Because of the way that cupping reduces excesses, it is better for people with excess syndromes - not just anyone with pain. You have to be careful with cupping to avoid weakening the person - which can happen if they are already deficient and it is used excessively.
All in all, cupping is an excellent modality for pain cases and most often leads to quick results with pain when accompanied with appropriate needling techniques and perhaps some tuina.