If I played Cavern of Souls, am I allowed to name a creature type for which there are no cards that exist for it? For example, Riggers? Taking it one step further, could I play Pithing Needle or Meddling Mage and name a card that doesn't exist?
Lastly, if I played Painter's Servant, could I name a color that wasn't one of the 5 colors in Magic? Could I choose purple?
Whenever you have to make a choice you are required to make a legal one. There are something like 200+ creature types in magic and you have to pick one of them for Cavern of Souls. Similarly with needle/mage you must pick a card that exists and is legal in the format you are playing. Painter's Servant is one of the 5 existing colors.
205.3e If an effect instructs a player to choose a subtype, that player must choose one, and only one,
existing subtype, and the subtype he or she chooses must be for the appropriate card type. For
example, the player can’t choose a land type if an instruction requires choosing a creature type.
You must choose a valid creature type. Some creature types only appear on tokens.
201.3. If an effect instructs a player to name a card, the player must choose the name of a card that
exists in the Oracle card reference (see rule 108.1) and is legal in the format of the game the player
is playing. (See rule 100.6.) If the player wants to name a split card, the player must choose the
name of one of its halves, but not both. (See rule 708.) If the player wants to name a flip card’s
alternative name, the player may do so. (See rule 709.) If the player wants to name the back face of
a double-faced card, the player may do so. (
So the named card must exist and be legal in the format.
105.1. There are five colors in the Magic game: white, blue, black, red, and green
...
105.4. If a player is asked to choose a color, he or she must choose one of the five colors.
“Multicolored” is not a color. Neither is “colorless.”
Lastly, if I played Painter's Servant, could I name a color that wasn't one of the 5 colors in Magic? Could I choose purple?
cEDH: [G(U/R) Animar] - [(U/B)(G/W) Redless Wheels] - [(G/U)(W/B) Redless Pod] - [(B/G)W Ghave Metapod]
You must choose a valid creature type. Some creature types only appear on tokens.
So the named card must exist and be legal in the format.
You have to make a legal choice here as well
cEDH: [G(U/R) Animar] - [(U/B)(G/W) Redless Wheels] - [(G/U)(W/B) Redless Pod] - [(B/G)W Ghave Metapod]
No, you have to name one of the five Colors of magic.
cEDH: [G(U/R) Animar] - [(U/B)(G/W) Redless Wheels] - [(G/U)(W/B) Redless Pod] - [(B/G)W Ghave Metapod]