Can you use signature spells in decks that don't include their associated legend, as long as the deck meets the color requirements?

Ruling: Signature cards can only be included in decks where you're running their associated Legend, even if you meet the color requirements. The exception is limited formats (sealed), where signature spells can be used outside their legend as long as the 2 colors are part of the 3 colors chosen for the sealed deck.

Nuances:

• In constructed formats, signature spells must be paired with their legend
• In limited/sealed formats, signature spells can be used independently if color requirements are met