Wall of Fire [Limited Edition Alpha] White its controller may discard a

its controller may discard a card that shares a color with that spell

If you search your library this way

Set: Kaladesh Promos Type: Legendary Planeswalker — Chandra Rarity: Mythic Cost: {2}{R}{R}

Set: Kaladesh Inventions Type: Artifact Creature — Construct Rarity: Mythic Cost: {3}{W}{W}

and his citadels lie empty

Wall of Fire [Limited Edition Alpha] White its controller may discard aSet: Limited Edition Alpha Type: Creature Wall Rarity: Uncommon Cost: {1}{R}{R} Defender (This creature can't attack.) {R}: Wall of Fire gets +1 +0 until end of turn. Conjured from the bowels of hell, the fiery wall forms an impassable barrier, searing the soul of any creature attempting to pass through its terrible bursts of flame.