Can you stack cards of the same number but different colors?
You may only play a single card each turn and note the rules mention playing "a card" not multiple. 

Rules


  You have to match either by the number, color, or the symbol/Action. For instance, if the Discard Pile has a red card that is an 8 you have to place either a red card or a card with an 8 on it. You can also play a Wild card (which can alter current color in play).