Placing a token on a card will do nothing unless there is another effect that interacts with it. For instance, placing a poison token on a character will not cause that character to be killed at the end of the phase, because there is no effect instructing you to kill it. However, removing a poison token placed by Tears of Lys or Tyene Sand (TS) will stop the delayed effect from killing the character.
You cannot place or remove shadow tokens from cards in shadows, because those cards are not considered to be in play. And even if you could, it would not cause those cards to gain or lose the shadow keyword.