Rule 202.2a

The five colors are white, blue, black, red, and green. The white mana symbol is represented by {W}, blue by {U}, black by {B}, red by {R}, and green by {G}. Example: An object with a mana cost of {2}{W} is white, an object with a mana cost of {2} is colorless, and one with a mana cost of {2}{W}{B} is both white and black.

Find this number in the current Rules