When your towel is brown after a shower it's often a result of bacteria and dead skin cells and even oil off your skin rubbing off onto the towel.
Body oils and skin cells are the most common reason for a towel to turn brown after a shower.
Towels can absorb the oil, sweat and dead skin cells from your body,
And over time, the substances can build up on the towel and lead to discoloration.
Having hard water or iron in your water could also lead to your towels turning brown after as shower.
Your water could have high iron content and then when you dry your body off the iron content from the water can get onto the towel and turn it brown as well.
Chlorine also breaks down towel fibers, making them stiffer and causing them to lose their whiteness, turning brown.
And if towels are not dried thoroughly after each use, the bacteria and mold can thrive in the damp environment of the towels and cause them to change colors and turn brown or other colors and lead to an unpleasant musty odor on the towels.
If your towel is wet, you can open it up so that the wetness of the towel can become dry.
And when you're using your towel to dry off, you're not only transferring water to the towel, but also some of your dead skin cells and other bacteria and germs even though you've showered.