Radiance

Radiance is an ability word associated with the Boros Legion of Ravnica. Radiance means "Effect on target [something] that affects all [somethings] that share a color with it." Radiance was introduced in Ravnica: City of Guilds and is only on red or white cards as those are the colors of the Boros Legion.

Radiance has a high Storm Scale rating owing to its lack of intuition and scaling problems. Its flavorful disconnect to the Boros may have led to unpopularity, adding to the issue.

Rulings

 * All creatures that share a color are affected, even your own.
 * A creature "shares a color" with any creature that is at least one of its colors. For example, a green-white creature shares a color with creatures that are green, white, green-white, red-white, black-green, and so on.
 * The color of a creature is determined by the colors in its mana cost. This is different from color identity.
 * If a radiance spell targets a colorless creature, it doesn't affect any other creatures. A colorless creature shares a color with nothing, not even other colorless creatures.
 * You check which creatures share a color with the target when the spell resolves.
 * Only one creature is targeted. If that creature leaves play or otherwise becomes an illegal target, the entire spell is countered. No other creatures are affected.