Colorless has two in-game meanings.
History[edit | edit source]
The idea of colorless mana was already introduced in Alpha with Basalt Monolith, Mana Vault and Sol Ring.  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).
However, the cost of many cards includes a generic mana cost (, and so on). What that means is you can use one of any mana, colored or colorless, to pay for it. Though you can use colorless mana to pay for generic mana costs, they are not the same thing. "Generic" is a type of cost. "Colorless" is a type of mana. You can't produce generic mana, nor could you have a colorless cost. The latter changed with Oath of the Gatewatch. 
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.  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. 
Colorless Mana Cost[edit | edit source]
While colorless mana being added to your mana pool is an evergreen mechanic, colorless mana cost is not. Colorless mana cost was first introduced in Oath of the Gatewatch and only appears in this one set.
Rules[edit | edit source]
References[edit | edit source]
- Mark Rosewater. (December 28, 2015 .) “A Solemn Oath, Part 1”, magicthegathering.com, Wizards of the Coast.
- Sam Stoddard. (January 8, 2016.) “Getting Colorless to Work”, magicthegathering.com, Wizards of the Coast.
- Matt Tabak. (December 28, 2015.) “Oath of the Gatewatch Mechanics”, magicthegathering.com, Wizards of the Coast.
- Alli Medwin. (January 27, 2016.) “The Costs of Costs”, magicthegathering.com, Wizards of the Coast.
- Mark Rosewater. (December 31, 2015.) "Do you all have any fear that now having to write (5) as CCCCC will take up too much space at card top and as such may affect the creation of spells and creatures with large mana costs?", Blogatog, Tumblr.