Chroma-jong Rules

This is the “Chroma-jong” rules, as designed by me. It’s a partial ruleset that replaces some normal tiles with special tiles (“chroma tiles”), and can be added onto most existing rulesets (e.g., Shuugi House Rules). It’s themed around the concept of limited wildcards (“tile interpretation”).

Special Tiles

This ruleset uses special (silver), (gold and red), (blue), and dragons (prismatic , fern-green , and light-siam ). By default, there is 1 copy of each chroma tile (see here).

Ruleset

The Gist (TL;DR)

1. each chroma tile gives +1 chip
2. silver are suitless
3. gold can interpret any one tile in the hand as any face after winning (the hand must remain winnable after interpretation).
4. red gives +1 han
5. melds containing blue are considered closed
6. if drawn during riichi:
   • prismatic can be interpreted as any face
   • fern-green can be interpreted as any “green face”:
   • light-siam can be interpreted as any “red face”:
7. the “Rainbow” is a yaku with variable value:
   • 5 chroma tiles: 1-han
   • 6 chroma tiles: 2-han
   • 7 chroma tiles: 3-han
   • 8 or more chroma tiles: yakuman

Clarification

“Face” vs “Chroma”

A tile has a “face” and possibly a “chroma”.

The face of a tile is the base suit+number/wind/dragon of that tile. I.e., and both have the face “5-pin”.

• the “wait” of a tenpai hand are described in faces, unrelated to chromas

The chroma of a tile is the special color of that tile beyond its face (if any). E.g., the chroma of is “red”.

• a tile’s special ability is determined by its chroma, unrelated to its face

“Interpret”

To “interpret a tile as X” is to treat a tile as if it has face X.

• interpreted tiles keep their original chroma (if any).
• multiple players can interpret a tile differently simultaneously.
  • example : a discarded silver tile can simultaneously deal into player A, who interprets it as , and player B, who interprets it as .

Due to the ability to interpret tiles, the following rules are different from regular riichi mahjong rules:

• a hand can have more than 4 tiles of the same face.
  • a hand is still considered tenpai even if it is waiting on a face that it already has 4 or more copies of
• “furiten” is caused by having discarded or skipped on any tile that can complete any interpretation of the current hand
  • example 1: having discarded regular , the player is in furiten with this hand: , where the has silver chroma
  • example 2: having discarded silver , the player is in furiten with this hand:
  • prismatic, fern-green, and light-siam chroma do not cause furiten if discarded before riichi or by other players after riichi (because they cannot complete the riichi hand when discarded in those ways).

Chroma Ability Clarification

1. silver : can be interpreted as any of
2. gold : after winning a hand, before calculating score, each gold tile allows the winner to interpret one tile in the hand as any face, as long as the resulting hand is still a complete hand with yaku.
   • this is mostly for getting takame and uradora. E.g., the winner interprets the in as to get iipeiko.
   • because the player has to win first, this ability doesn’t change the player’s wait, cause the player to be furiten, or change how the player won (tsumo vs ron)
3. melds containing blue still follow all rules of a meld. The only difference is that now the meld is treated as closed for the purpose of determining eligibility for riichi, fu, and yaku.
4. the value of the “Rainbow” yaku changes based on the total number of chroma tiles, not the number of unique chromas.
   • see Rainbow Stats for Nerds

Modifications

Here are some suggested modifications of Chroma-jong to try:

1. adding or removing copies of the chroma tiles. The value curve of the “Rainbow” yaku should also be adjusted.
2. making the light-siam only interpretable as to reduce the memory complexity.

Rainbow Stats for Nerds

The value of the “Rainbow” yaku is estimated from a hypergeometric distribution of “drawing X chroma tiles within 31 draws out of a 136-tile wall that contains 15 chroma tiles”.

I picked “31” as the draw size because players draw 30.5 tiles if no calls were made before an exhaustive draw. The real probability is likely similar, because while players can get more chroma tiles from calling discards, the chroma tiles aren’t easily discarded and most “vanilla-ruleset” games also end around 25 draws (12 turns) (according to MajSoul Stats, for Jade games)

Also, being able to draw X chroma tiles is much easier than being able to win with X chroma tiles. As X increases, being able to win with X chroma tiles drawn also gets harder faster than being able to draw X chroma tiles. In fact, reaching 10+ chroma tiles means you have a VERY specific hand. E.g., uses 12 chroma tiles, and this should be the highest number of chroma tiles you can have in 1 hand.

That said, the probability of the aforementioned hypergeometric distribution is as follows:

• P(X≥4)=46.00%. I.e., about 46% of time, a player gets at least 4 chroma tiles out of 31 draws. While winning with 4 chroma tiles is harder, it may be still too common for this to be a yaku.
• P(X≥5)=21.04%. “5” may be a good minimum chroma count for the yaku.
• P(X≥6)=9.148%. Not much to say here.
• P(X≥7)=2.748%. It’s now very hard to get this many chroma tiles, let alone use this many in one hand. However, it’s still a bit easy compared with even the most common yakuman (the probability of suuanko is around 0.05%).
• P(X≥8)=0.626%. Considering how hard it is to fit 8 chroma tiles in one hand, this is worthy of a yakuman. Its real winrate may end up being closer to rarer yakuman like tsuuiiso, which is around 0.005%.

Images

Properly customized tiles:

Viable setup (just mark each non-red chroma tile, and use the original red fives as the chroma red fives):