What happens to the current color when you successfully challenge a Wild Draw 4?
If the Wild Draw 4 is challenged the person who played the card must show their hand to the challenger. If there is a card in their hand that they could have played then the player must draw 6 cards. If the person who played the the draw 4 has no cards that they could have played then the challenger must draw 6 cards. Regardless the outcome of the challenge the Wild Draw 4 stays in play and the game is played normally. All of this info can be found in the rulebook that came with the cards.