The colorless mana symbol
Colorless has two in-game meanings.
History
The idea of colorless mana was already introduced in Alpha with Basalt Monolith, Mana Vault and Sol Ring.[1] It set the premise that most mana comes in one of the five colors (white, blue, black, red, or green), but there is a sixth subset of mana that is not any color. This mana can only be used to pay for costs that don't require color (for example artifacts).
In early sets, the text for Sol Ring was still spelled out as "Add 2 colorless mana to your mana pool". However, from Odyssey onwards, this was officially changed for editing reasons to "Add 
to your mana pool".
The cost of many cards includes a generic mana cost (e.g., 
, ), meaning that a player can use that number of any mana (colored or colorless) to pay for it. Though one can use colorless mana to pay for generic mana costs, they are not the same thing. Generic is a type of cost, whereas colorless is a type of mana; a player cannot produce generic mana and there are no colorless costs. The latter changed with Oath of the Gatewatch.[2]
R&D came to the conclusion that they had to stop using the mana circle with a number to represent both generic and colorless mana. Changing the representation of colorless mana was easier, as it's represented far less, so they to made the new mana symbol 
that represents one colorless mana.[3] In our example the text for Sol Ring now is "Add to your mana pool". One of the consequences of introducing a colorless mana symbol was the change to over 300 cards in Oracle, the central database of official card wordings.[4]
Colorless mana cost
While colorless mana being added to your mana pool is an evergreen mechanic, colorless mana cost is not.[5] Colorless mana cost was first introduced in Oath of the Gatewatch and only appears in this one set.
Rules
From the Comprehensive Rules (April 12, 2024—Outlaws of Thunder Junction)
- 107.4c The colorless mana symbol {C} is used to represent one colorless mana, and also to represent a cost that can be paid only with one colorless mana.
From the Comprehensive Rules (April 12, 2024—Outlaws of Thunder Junction)
- 106.10. If an effect would add mana represented by a generic mana symbol to a player’s mana pool, that much colorless mana is added to that player’s mana pool.
From the Comprehensive Rules (April 12, 2024—Outlaws of Thunder Junction)
- 202.2b Objects with no colored mana symbols in their mana costs are colorless.
References
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