Historical Rules · 3 May 2019
Rule 105.2
Official source text from the 3 May 2019 Rules release
An object can be one or more of the five colors, or it can be no color at all. An object is the color or colors of the mana symbols in its mana cost, regardless of the color of its frame. An object’s color or colors may also be defined by a color indicator or a characteristic-defining ability. See rule 202.2.