Well, I know people with fantasies will tell you that color shades represent different problems. He is right, color depth does represent some problems, but his understanding of diseases is wrong, and color depth does not represent any diseases. It just shows that when cupping, the air tightness between the jar and the skin is different from the amount of burnt air, and the cupping time is also different. The better the air tightness, the more oxygen consumed by combustion, the longer the cupping time and the darker the color. Conversely, the lighter the color.
From my own observation, the color of cervical vertebra and coccygeal vertebra is the lightest. Why? Because the surface of these two places is the most uneven and prone to air leakage. No matter you or others, as long as they are not fat, they are the lightest in those two places. This is the most basic knowledge of physics and physiology, which needs no explanation. Cupping is the most difficult on the spine, because it has the least fat, tight skin and is the most prone to air leakage.
Of course, if you are willing to believe in fantasy, you are happy.