In Uno, if I play a Wild Card and pick a color, can I also play a card in that color?
No, you can only play one card per turn.

Let’s Play


  On your turn...
  Example: If the card on the Discard pile is a red 7, the player must put down a red card OR any color 7. Alternatively, the player can put down a Wild Card...


Functions of Action Cards


  Wild Card – When you play this card, you get to choose the color that continues
  play (any color including the color in play before the Wild card was laid down)...


Here's a great place to read the rules for yourself:
From mattel.com: UNO Rules 2018