There are a couple reasons of dim armpits, elbows and knees show up, and it is on the grounds that those body parts regularly accumulate the most earth and sweat, and thus they turn dim over a time frame.
Here are some tips:
Lemon
Simply squeeze the juice of a fresh lemon and apply it on your elbows, knees, and underarms.
Let it dry for 15 minutes and rinse off with cool water.
Do this 2-3 times a week.