Coin Discoloring

Coin discoloring is an intentional design of the Personalisation A.I. closely related to Dynamic Level Rearrangement. They will appear based on the players actions and skill level. These discolorations are mostly consistent in their behavior between personalized and will either make the game less or more challenging.

Coin Types
Note that these are the most common manifestations of discolored coins, and may act differently than listed.

Green
Green coins oddly don't count as coins, as they do not increment the coin counter. However, they will still increase health, with the unique amount of 3 health per 1 coin. They often appear in easily accessible areas, although sometimes drop from particularly difficult enemies. It is believed the Personalization A.I. will create these if the player is performing poorly, such as taking frequent damage or being at low health often.

Orange
Orange coins are smaller than yellow coins. When collected, they will give 0.5 coins. However, they also restore 2 health, more then a regular yellow coin. The A.I. will spawn these if the player is doing somewhat poorly.

Purple
Purple coins are nicknamed "Poison Coins" due to their behavior. As opposed to increasing Mario's health, they will hurt him for 1 point, and sometimes subtract from his coin counter, usually taking one coin although have allegedly taken up to 10. They usually appear in places the player is likely to collect them without warning, such as from boxes that have no indication of the contents.

Clear
Clear coins appear invisible or only showing the black star outline, although they will appear fully visible as coins, typically blue, after contact. As opposed to spawning on level loading, players say they usually appear in areas that have already been explored. They are believed to be a reward from the A.I. for thoroughly exploring levels.

An alternate manifestation of Clear Coins is that they are any of the coin types but can only be interacted with using the Vanish Cap. It is unknown why these are added, though it is possible they are used to make caps be used more often.

Pink
Pink coins are larger than yellow coins, and give 10 coins when collected as well as a small period of invincibility, along with a full health refill. They usually appear when the player is doing poorly. They are also said to appear in hidden areas that are hard to find as a reward for exploring.

White
White coins appear around the same size of blue coins. When collected, they usually do nothing and have reportedly crashed the game on rare occasions. Notably, white coins are actually in the game's files as the standard coin sprite, with the coin color being changed depending on which coin object is used.

Metal
Metal coins appear around the size of blue coins, and appears to use the Metal Cap texture. It acts similarly to a box instead of a coin. Upon walking into it, it cannot be collected. However, if it is thrown, it can break into any form of coin. It can also be broken when walked into while using the Metal Cap. Similarly to the secondary manifestations of clear coins, it is believed these coins were intended to be used to increase usage of the caps.

Cyan
Cyan coins are normally as large as a normal coin. Once a player gets close enough to one, the Cyan coin begins linger in front of the player. After the player chases it enough, the coin can be collected and grants 10 coins. Cyan coins share a base behavior with an unused function in the game for the red coin, which makes it hover in front of Mario at a set distance; However, it's most likely that the Personalisation A.I. is only reusing an unused function in the game, and there was never a coin with that function intended.

Rainbow
Rainbow coins are 2 times the size of a normal coin. When a player collects it, it fills your health, gives you 10 coins and fires a "Flare" which shows the location of a star. The A.I usually adds these if a player is doing very poorly to the point it's intentional. The rainbow coin was likely scrapped due to it being incredibly OP.

A much more rare version of rainbow coins is ones that only spawn after you get 120 stars, along with that five rainbow coins will spawn in each level at the size of a blue coin. These five coins will normally be scattered all around the level making them very hard to get. After you collect all five of them a star will spawn. On the star select screen it is right below the 100 coin star. It’s also been reported that these coins can spawn in anomalous levels all though the truth of these reports is unknown.

Dark Green
Dark green coins are the rarest coin to spawn. They only spawn if the player has 120 stars, has collected every coin in the game (not including the impossible ones), and has a total of 64 to 120 play sessions or more. The only course dark green coins spawn in is wet dry world, and is found in the flooded town. They are easy to find there. The behavior can vary depending on your copy. Some players have reported it spawning a 121'st star when collecting them all in the town. Some other players have reported them giving them 999 coins, along with infinite lives when collecting 5 of them. One reported the coins not being collectable at all, and instead spawning you into a beta version of WDW.

Sky Blue
Sky Blue coins are a light blue color and can sometimes replace golden coins only in Princess Peach's Castle. They can be found throughout the inside of the castle (most commonly replacing the two coins on the mezzanine of the first floor) and the castle grounds. They can also be dropped from Boos killed in the castle courtyard. They cannot increase the coin counter.

Theorizing
The fact that green, pink, orange, cyan and purple coins deviate so far from regular coin behavior has lead some to believe that they are placeholders for objects that where cut from the final game.

The fact that the main manifestations of clear coins are strikingly similar to hidden coins from later Mario games has lead many to believe that the idea was reused for the later games.

Dark green coins were most likely placeholders for a scrapped rare collectable object that was most likely found in WDW.