Colorless mana costs for a card can only be met with sources that produce colorless mana, and cannot be met with any other colored mana source, compared to "generic" mana costs which can use any colored or colorless mana source
Regardless, as a 5-color commander you may play any hydra youd like, including any hydra on this list
If you have creatures like Goldspan Dragon or care about artifacts (such as branching into UG or RG) then Provisioner is a slam dunk include
Condition: Brand new, factory sealed, never opened
Mill Mill isnt very common to see at a 4-player table where deck sizes are at least 100 cards, but there are some commander's deck that relies on milling as a win condition
Schematic Artifacts All 63 of the retro artifacts also have a schematic form