Red Coin

Red Coins are coins that sometimes appear along with Yellow Coins in the Mario series. The only difference between a yellow coin and a red coin is that red coins are worth 2 coins while yellow coins are worth one coin, and when a player gets 8 of them, they recieve a 1-Up Mushroom, Power-up, etc.

Super Mario World 2: Yoshi's Island
Red Coins were first seen in Super Mario World 2: Yoshi's Island, in which they were hidden behind regular Gold Coins. Red coins tend have a slightly red tint to them, but this cannot be a guaranteed way of finding out, as some Gold Coins also have a red tint and vice-versa. The only way to tell for sure which Gold Coins hid Red Coins was to grab them, or use a Magnifying Lens. There were 20 Red Coins in each level, and grabbing one added one point to Yoshi's score tally at the end of the level.

Super Mario Bros. Deluxe
Red Coins were once again seen in Super Mario Bros. Deluxe. In certain missions they (along with Yoshi Eggs) had to be collected to complete that mission.

Super Mario 64
Red Coins were later seen again in Super Mario 64 and Super Mario 64 DS, in which each one was worth two Yellow Coins. Collecting all 8 Red Coins in one level would make a Power Star appear over a Star Marker. In Super Mario 64 DS, Bob-omb Buddies have the ability to detect red coins when the player talks to them.

Super Mario Sunshine
Red Coins make another appearance in Super Mario Sunshine. Similar to Super Mario 64, collecting all eight in a level would make a Shine Sprite appear. However, Red Coins did not affect the player's total amount of Gold Coins and were treated as a completely different currency altogether.

Secret Levels
In addition, red switches are located at the starting platform of Secret Levels, and when the player Ground Pounds the red switches, eight Red Coins will appear and the timer will be activated. Mario must then collect all 8 Red Coins before time runs out or he will lose a life and must restart the level.

New Super Mario Bros.
In New Super Mario Bros., there were Red Rings in several levels, and if Mario or Luigi jumped or swam through one, eight Red Coins would appear. If they collect them all in the time limit, they are rewarded with a power-up. If the player had already got a power-up worth more than a Mushroom (i.e. a Fire Flower), then instead, they would get a 1-Up Mushroom instead of a power-up.

New Super Mario Bros. Wii
Red Coins and Red Rings also make a return in New Super Mario Bros. Wii. They serve the exact same function as in the preceding game and they will still give the player a power up if all coins are collected. However, small Mario collecting the coins would make a Fire Flower (or other power-ups of a similar power) instead of a Mushroom. Like before, if the player had already got a Fire Flower or similar power-up, they would get a 1-Up Mushroom after collecting all the Red Coins

Mario Hoops 3-on-3
Red Coins appeared in Mario Hoops 3-on-3 under the name 10 Coins. They can be found in ? Panels, and as evident by their name are worth ten normal Coins. They are only found if the player manages to combo the dribbling of acquiring the coins without being interrupted.

Yoshi's Island DS
In the sequel to Yoshi's Island, Yoshi's Island DS, Red Coins worked the same way as they did in their first appearance; however, the Magnifying Lens was absent in that game. If the player can collect all the red coins in the level, the player will get a better score upon completing the level.

Mario Party DS
In Mario Party DS, Red Coins replace Coin Bags in bonus mini-games unlike the previous versions of the Mario Party games. They are, like Coin Bags, worth 5 Coins. They are obtained like Coin Bags too; rarer and sometimes harder to acquire.

Trivia

 * In Super Mario Sunshine, the symbol on the Red Coin slightly resembles Barry.