One of the primary functions of your skin is to be impermeable to most of the stuff that’s outside your body. It’s not perfect, but it’s pretty effective.
We notice the few pigments that leave a lasting color on our skin because they’re rare and they have a visible effect. Most of us walk around most of the time in our natural skin color, even though we’re frequently exposed to colored liquids. So really it’s more a question of why anything is able to discolor our skin by surface contact.
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