Game play in Coloretto is simple: Either draw a card to play to a row; or take a row of cards to add them to your collection. A row can have at most three cards; so at some point everyone is forced to take a row. Once all the rows have been claimed; players start a new round; drawing or taking once again.
What are you trying to do with these cards? Collect huge sets - but only in three colors as every color beyond the third will cost you points. Jokers are highly-prized as they always match what you want; and +2 cards provide sure points; giving you a back-up plan if everything goes south in terms of the colors you're collecting.
Once only a few cards remain in the deck; the round ends and everyone tallies their score; choosing three colors of cards to score positively while any other colors count against you. Each color is scored using a triangular number system: the first card in a color is ±1 point; the second card is ±2 points; and so on. The player with the high score wins!
A two-player variant is included with some versions of Coloretto; with the only change being that rows can have only 1-3 cards placed in them; depending on the icons on the row cards.