Uno Online, also known as 4 Colors, is a virtual version of the original Uno card game. It provides turn-based discard play, which is also sometimes referred to as turn-based "shedding" of cards. The player's goals are to shed their cards to win each round and earn 250 points before their opponents. The game assigns points at the end of each round by adding up the numbers on the cards remaining in their opponents' hands. The player can choose to compete against two, three or four bots.
The game starts with an invisible dealer assigning seven cards to each player. The dealer places a face-up card next to the draw pile at the center of the table. The player must shed a card from their hand during their turn or draw a card. To shed one, they must use a card that either matches the number on the face-up card or its color. If they have only one card remaining, they must quickly press a "1" button to alert the other players. If they fail to press it quickly enough, the dealer penalizes them by adding two new cards to their hand.
Uno Online uses special cards to enhance play and make finishing a round more difficult for all players. If any player receives or draws a 50-point Wild Card or a Wild Draw Three card, they can change card colors during their turn. With the second card, they can also penalize the player after them by forcing them to add three cards from the draw pile to their hand. Three 20-point special cards (Action, Skip and Reverse) change play in other ways. With Action, the player after the one who uses it must add two cards to their hand from the draw pile without shedding a card. With Skip, the next player loses their turn. With Reverse, the previous player makes the next move.
