What happens If you challenge a player who say nominates red with his/her wild draw 4 card and you challenge successfully, then the person who played the card would pick up 4 cards instead of you. My question is, does the colour stay as red in this situation or to the colour just before it was reds?
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.
The person who willfully plays a wild draw 4 card to change the color when they had an obvious card to play, is to discard their hand and go watch TV or something, as they have broken the first rule of playing games: No cheaters. Uno is not a bluffing game, and cheating is not expected or tolerated.
--But perhaps I'm being a bit sensitive.
That said, we usually just have the miscreant pick up their wild and play the appropriate card, as our rule breakers are normally the younger kids who didn't recognize the valid card to play.
(Edit) The rules stat that if challenged, and player had a valid card to play, they forfeit their turn. I don't see any rule addressing what happens to the Wild+Draw 4. But if a player has to forfeit their turn, I would think that you would continue play and their card was never played. They would need to pick it up, since they can't play any cards.. they forfeited.