Here is what I figured out by now (I mostly use dry cupping):
In my oppinion, the intensity of the vacuum is very important, as it decides what layer of the body are we working with. And to use cupping as a diagnostic tool, it is vital that we first work on the surface and slowly advance into the depth. If we start by working with deep cupping, the first marking shows us states of the surface and deep tisue, rendering the marking very hard to read.
Besides the color, I also observe the shapes of the coloured surfaces. When the surface is equally coloured, the flow is steady. If the surface is pale, we have deficiency in the underlying channels/structures. If the surface is dark red or darker, we usually have blood stagnation patterns. Ideally, the whole surface gets equally bright red fast, then dissapears to the normal skin color in matter of minutes.
Dots: Mark of any color covered with dark dots indicates dry patterns and/or mild qi stagnation.
Discolorations: Mark that is only partially coloured or has segments of different colors, indicates Spleen deficiency or Phlegm invasion. These markings are very simmilar and connected to cellulitis.
The puffyness of marks: Some marks return to their normal state as soon as we remove the cuppings, while others retain their shape (in my language, I call them cakes :P). This to me indicates severe/chronic Phlegm and low blood circulation on the surface of the body, especially if the marking remained pale.
There is also the pain factor, which would be much harder to dissect, as pain is so subjective. Generally speaking, more pain means more surface tension, indicating stress along the surface channel/s. No pain or dull pain can then mean internalized problems, chronic conditions and/or nerve disharmony. It is also interesting to observe the passing of the pain. Some clients start by feeling a lot of pain, which recedes fast, while others keep with the pain for the whole duration of the cupping session. Rarely, a person feels little pain at the start, which then intensifies.
I would love to hear comments and more opinions on this :)