|Yellow and Blue Coins
Red Coins are red-colored coins similar to the yellow coin. These coins have twice the coin value of a normal coin, and appear in groups of eight in every normal and hidden course in the vanilla game (excluding Peach's Secret Slide). When the eighth Red Coin is collected, a red coin star has a four-frame delay before it spawns at the star marker. Note that spawning a 100 Coin Star has no delay.
When a red coin spawns, it checks for the nearest normal hidden red coin star and sets that to its parent if it exists. If none were found, it looks for the nearest Bowser hidden red coin star and sets that as its parent instead. If both of these checks fail, its parent is set to NULL. After setting its parent object, the object is given its hitbox.
From here, the coin's only behavior is to respond interaction from Mario. If it is interacted, it will firstly increment it's parent coin count as long as its parent is not NULL. On the Japanese version, it will spawn a normal coin collection sound at this point. In both the US and JP version, it next spawns an orange number as long as the coin is not the eighth collected (More specifically, is not the coin which would normally trigger a star. It is possible to have more than eight coins in a hacked ROM). On the US version, it now instead spawns its sound, which increments its pitch for every red coin collected.
The coin finishes by spawning coin sparkles and despawning.