As long as Omnath, Locus of All remains under your control, you'll retain unspent mana as steps and phases end, although that mana will become black. This means you can add mana and spend it during a future step, phase, or turn. Once Omnath leaves your control, you'll have until the end of the current step or phase to spend the mana before it is lost.
The mana symbols in the card you reveal may be in any combination of colors. For example, you could reveal a card with mana cost {1}{R}{R}{R}, one with mana cost {4}{U}{R}{W}, one with mana cost {W}{U}{B}{R}{G}, or one with mana cost {R/G}{G}{G/W}.
If unspent mana you have has any restrictions or riders associated with it (for example, if it was produced by Omen Hawker), those restrictions or riders remain associated with that mana when it becomes black.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Helge C. Balzer
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Omnath's last ability isn't a mana ability even though it can cause you to add mana. It uses the stack and can be responded to.
