There’s a real difference between clamp discoloration and Vacuum discoloration
Clamp discoloration is from the lack of oxygen in the cells which can be replaced rather quickly if it hasn’t gone on to long (hence the 10 min. rule)
Vac hanging is really different. Here your getting a breakdown of the blood vessels and the discoloration is from the wrongful disbursement of blood. Once that blood is trapped it takes a very long time for it to go away.
Discoloration in the shaft skin from clamping is very much the same but for a different reason. When you clamp your cutting off the flow of oxygenated blood from the cells and this causes the blood cells to turn dark. If these blood cells become trapped in the dermal layer of the skin they are very much like a tattoo.
The difference between vac hanging as opposed to vac pumping is when hanging all the vacuum is concentrated on the glan and tip, where with vac pumping it gets spread out to more tissue area up and down the shaft.
Some guys get color blotches. Others get fluid blisters. I get blisters.
I guess the fluid blisters can be prevented but the color blotches can’t. I hate surprises when I take it off, so I quit vac usage.