My opponent plays a vampire deck and has Vampire Nocturnus out. It says
Play with the top card of your library revealed. As long as the top card of your library is black, Vampire Nocturnus and other Vampire creatures you control get +2/+1 and have flying.
The top card of their library is a swamp. Does the swamp count as a black card (giving their vampires +2/+1 and flying), or is it colorless (meaning the vampires remain unbuffed)?
The color of a card is determine by its mana cost and by its color indicator.
As you can see from this search of the complete database of Magic cards, no land has a mana cost, and only one land has a color indicator, so all lands are colorless except for that one land, Dryad Arbor.
202.2. An object is the color or colors of the mana symbols in its mana cost, regardless of the color of its frame.
202.1. A card’s mana cost is indicated by mana symbols near the top of the card. [...]
- 202.1b Some objects have no mana cost. This normally includes all land cards [...]
202.2e An object may have a color indicator printed to the left of the type line. That object is each color denoted by that color indicator. (See rule 204.)
Note that Commander has the separate concept of a card's color identity, which is not to be confused with a card's color (despite the names). Color identity counts all mana symbols appearing anywhere on the card. This concept is exclusive to commander, though, and while a swamp has a color identity of black, Swamps are still colorless.
