This is for Commander play. The General's mana defines what other card you get to use, BUT what happens when you have a colorless General? Can you play any other mana cards (WUBRG), OR are you stuck playing ONLY colorless.
You can only use cards with a colorless color identity with a commander that has a colorless color identity.
Rule 903 has the rules for the commander format, and the following subrules are relevant here:
903.4. The Commander variant uses color identity to determine what cards can be in a deck with a certain commander. The color identity of a card is the color or colors of any mana symbols in that card’s mana cost or rules text, plus any colors defined by its characteristic-defining abilities (see rule 604.3) or color indicator (see rule 204).
903.5. Each Commander deck is subject to the following deck construction rules.
- 903.5c A card can be included in a Commander deck only if every color in its color identity is also found in the color identity of the deck’s commander.
"Colorless" color identity means that the card has no colors in its color identity. If you tried to put a card in your deck that had any colors in its cost or rules text or indicator, then it would have a color in its color identity that is not in the commander's color identity.