: Add , , or . Spend this mana only to cast a Knight or Equipment spell.
Mana produced by the second ability of Tournament Grounds can't be spent to activate abilities, including equip abilities.
If an effect refers to a “[subtype] spell,” it refers only to a spell that has that subtype. For example, Knights' Charge is a card with “Knight” in its name and benefits Knights, but it isn't a Knight card.