Are bonus sheet cards allowed to be pie breaks, since they don’t affect format legality? I get why desert twister wasn’t in thunder junction or the big score, but was very surprised it didn’t appear in breaking news or the commander decks

There are other rules about color pie breaks beyond the “we don’t add them to formats that don’t have them” rule. Another one is “we try not to reprint them if there isn’t a lot of player demand for the reprint”. Yes, some are staples in their format, so there’s a demand, but we try to reprint color pie breaks that we don’t have to.

Are there any abilities that aren’t in pie for any singular color?

Yes, a bunch require multicolor options. The most famous is probably “destroy target permanent”. (Desert Twister is a break.)

when you fight against a decision (eg Morph not using the stack, Desert Twister being included), what tactics do you find yourself gravitating towards? e.g. do you launch into arguments? do you try to find common ground first? do you promise to give something they want in return? do you bribe with home-made cookies? etc

Usually, I explain the larger philosophy behind my decision.

Hey Mark, are there things that can only be done when the colors combine? Like, are there any multicolor-only mechanics?

“Destroy target permanent” is only doable in monocolor. And yes, Desert Twister exists, but it shouldn’t.

Hi Mark. White can destroy any permanent type, but usually individual cards restrict what permanents that card can target. Do you think white could get a color-shifted version of the (ludicrous) card "Desert Twister"?

White can’t pinpoint destroy lands.

"Being a color pie break requires you being in the color, not whether or not a deck can play the card." (From an earlier answer.) This is something that took me a long time to understand, and I'm still not sure I fully grok it. Desert Twister and Scour from Existence both let green remove permanents, but only Twister hurts the game. So it's more than an issue of competitive balance. It's as if the colors have personalities, and color pie breaks are out-of-character actions. Is that at all close?

We set limits for colorless abilities. For example, it costs 7 to destroy a permanent. This keeps colorless cards from undermining the color pie. Color identity is more than just what can go in the deck, but what that color itself can do. If we printed colored cards doing off color things at colorless limits, it would set bad messages about what the color can do. Most players aren’t good at telling rate, so a colored card at a bad rate just says, “Yeah, this color can do this.”

[You:] "Learning Yanggu travels with Mowu is not meant to be some betrayal..." [Also you:]The best way to show that a colour is weak at something is to never print cards that allow it to do that thing; a single colour pie break will be cited as a precedent. (eg, Desert Twister; timeshifted sheets) // Regardless of intent, Mowu is currently the Creative equivalent of a colour pie break unless Yanggu is able to create planar gates.

Planeswalking is different from planeswalker to planeswalker. Some planeswalkers can planeswalk quickly and some take longer. Some can planeswalk often and some can only planeswalk infrequently. Planeswalking rules are not hard and fast. They are default rules. Most planeswalkers can’t do thing X, but that doesn’t mean no one can do them. It means it’s a rarity. That’s why the comparison doesn’t hold out. The color pie has crisp delineations because it’s all about how the colors are different from one another. The planeswalkers, in contrast, specifically vary from person to person. Each one is unique. So seeing what forty planeswalkers can do will give you a general sense of how the rules roughly work, but it’s not a definitive answer. They are two different systems, one built on delineation (color pie) and one built on variation (planeswalkers).

"How bad of a break is beast within and how likely we see anything like it againIt’s bad and we shouldn’t see it again in monogreen." I presume Desert Twister is out of the question too?

Absolutely.

I feel your points on mr-obliterator's ask are valid, but doesn't having overcosted/bad downside enable colors to still have their weaknesses and bended color pie cards? I'd posit that Desert Twister is fine but Song of the Dryads isn't since DT is prohibitively expensive, or Gamble not breaking red since it's downside doesn't make it Demonic tutor but in red.

Dessert Twister is out of color pie. If you want a weak way to do something in a monocolor deck, go to artifacts. That’s part of why they exist.

Are there any individual effects that can only be done by combinations of colors that can't be done any individual color?

Yes. “Destroy target permanent” can be white/black or black/green but isn’t allowable in any monocolor (in modern color pie - yes, I’m aware Desert Twister exists).