If you are talking about making it darker, more UV tanning. Discoloration is a process of blood breaking and pooling in your skin. This is only temporary. Pumping at high pressure for longer period, rather than 5 minutes, you go for 15 continuous minutes or more, allows blood and plasma to pool in the epidermal layer, causing discoloration. It goes away after a few day, few weeks, or a few months from repeatedly adding more discoloration session.
Same for UV tanning. It darkens, but goes away in a short while. The only quasi darkening process is through tattoo inking using shallow injection. Tattoo artists know how to do this. A friend of mine actually made his penis looks like an elephant trunk 15 years ago, and it still has darken color to this very day. Sorry for the poor imagery description.
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