In Uno when playing a Wild Draw 4 card, can I choose a color not in my hand?
Yes you can do that. In the rules, there is nothing restricting the color called.

Wild – This card represents all four colors, and can be placed on any
card. The player has to state which color it will represent for the
next player. It can be played regardless of whether another card is
available. If turned up at the beginning of play, the first player
chooses what color to continue play.
Wild Draw Four – This acts just
like the wild card except that the next player also has to draw four
cards as well as forfeit his/her turn. With this card, you must have
no other alternative cards to play that matches the color of the card
previously played. If you play this card illegally, you may be
challenged by the other player to show your hand to him/her. If
guilty, you need to draw 4 cards. If not, the challenger needs to draw
6 cards instead. If turned up at the beginning of play, return this
card to the Draw pile, shuffle, and turn up a new one.