There are only five colors in magic. However, there are many instances where mana of any color can be used to pay certain costs, several cards that produce mana with no color identity, and a few special instances where mana must have no color at all. It is even possible to build a deck that consists entirely of colorless cards!
Many cards feature a number in a gray circle, indicating that mana from any source may pay that portion of the casting cost. This mana is called "generic mana," and it need not be colorless, but colorless mana can be used to pay these costs. The most common colorless cards are artifacts, and these almost all have casting costs that are entirely generic mana. As such, they can be used to modify any deck. Artifacts can provide passive benefits, represent mechanical or magical construct creatures, enhance creatures with special equipment, or now with the release of the Kaladesh block, offer vehicles for your creatures to control in battle.
Another colorless card type that has occurred in a few sets is the Eldrazi. These cards represent the Lovecraftian creatures that are imprisoned on the plane of Zendikar, and they seek to escape to devour the multiverse in the backstory of the game. In game, they represent powerful creatures that produce especially destructive effects.
Prior to the Battle for Zendikar block, cards that produced colorless mana used a number in a gray circle identical to the symbol for generic mana in the casting cost of spells/abilities to state the quantity of colorless mana produced. However, the new cards included some that required a portion of the mana cost to be specifically paid by a colorless source. Thus, a new symbol was added to the game:
This clarifies a potentially-misleading concept, and while many older cards are in circulation and will see play for the foreseeable future, understanding the distinction between generic mana and colorless mana will help you better understand the game regardless of whther the new symbol is present or not.
Remember, you can trade physical cards online to perfect your deck using Pucatrade! (2026 update: dead link)
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