Historical Rules · 1 February 2010
Rule 903.4c
Official source text from the 1 February 2010 Rules release
A card may be included in an EDH deck only if each mana symbol contained within its mana cost or rules text is colorless or is a color or colors that's in the general's mana cost. Example: A deck has a white-blue general. Each mana symbol in each card in that deck must be white, blue, white-blue hybrid, or colorless.