Gold Mario

Gold Mario and Silver Luigi are forms debuting in New Super Mario Bros. 2, acquired by grabbing a Gold Flower. In this form, Mario can shoot big Gold Fireballs and Luigi can shoot big silver fireballs, defeating any enemy or breaking any block that is within a certain distance or any that they touch, turning them into coins. In addition, the shockwave will also empty out several non-brick blocks/brick blocks that are secretly containers. This form has only appeared in Nintendo 3DS games.

New Super Mario Bros. 2
Gold Mario debuted as a power-up form in New Super Mario Bros. 2. In this form, the character becomes a glowing statue made of gold. In Luigi's case, he becomes a glowing statue made of silver. In both cases, the form resembles Mario's metal form since gold and silver are a type of metal. If Mario/Luigi reaches the Flagpole in this form, he receives a coin bonus. The higher the pole is grabbed, the more coins he will receive. Gold Mario/Silver Luigi reverts into Fire Mario/Luigi upon clearing a stage, making it impossible to keep this power-up between levels. Gold Mario is able to defeat almost every enemy in the game, including Boos, Thwomps, and even Spiked Balls (though not giant ones), rivaling the power of Hammer Mario. He can also defeat bosses, but they take more than one hit to be defeated (but not Bowser). The power-up, however, cannot defeat the Boohemoth.

Mario Golf: World Tour
Gold Mario appears as a separate downloadable character in Mario Golf: World Tour. He has the same stats, voice, and animations as the actual Mario with the only different one being his Double Bogey animation, where he turns into Super Mario instead of Small Mario. He has a permanent Gold Flower shot, meaning he collects a coin for every yard the ball travels. This means he can not use special shot types (Power, Item, or Approach) and is not allowed in Coin-Collecting Tournaments or challenges. He also cannot be played against in match play, but may be played as instead. To obtain Gold Mario, all three downloadable packs, the Mushroom, Flower, and Star, must be bought.

Super Smash Bros. for Nintendo 3DS / Wii U
In the Golden Plains stage in Super Smash Bros. for Nintendo 3DS, if a character collects 100 of the coins scattered across the stage, they will turn gold. While in this form, the character's attack power and launch resistance is increased.

Trivia

 * In the first Animal Crossing game, there are two furniture items based off of Mario and Luigi, the Mario Statue and the Luigi Statue. The Mario Statue is actually golden, and the Luigi Statue is silver.
 * The model for the gold/silver fireballs always spins clockwise, even when thrown to the left.
 * Gold Mario's artwork for Mario Golf: World Tour is very similar to Metal Mario's from Mario Kart 7.