Morophon, the Boundless has a mana cost is 7 colorless mana, but he has an ability that includes all mana types. If he is my commander, what cards can be in my deck?
The color identity of your commander includes mana symbols in both the cost and any abilities; so you would be able to use cards of any color if your commander is Morophon.
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).
903.5c 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.
And similarly, Morophon could only be in your deck (as not your commander) if your commander contained all 5 colors in its color identity.