Void Mirror 
Artifact
$0.37 #15611 1.00 5.3 years
Whenever a player casts a spell, if no colored mana was spent to cast it, counter that spell.
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Cards similar to Void Mirror expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Vexing Bauble96%
$2.17↑#1304↑0.51↓2.2 years↓
"Whenever a player casts a spell, if no mana was spent to cast it, counter that spell."
Boromir, Warden of the Tower89%
$6.61↑#851↑0.60↓3.2 years↓
"Whenever an opponent casts a spell, if no mana was spent to cast it, counter that spell."
Lavinia, Azorius Renegade88%
$0.18↓#3183↑0.917.6 years↑
"Whenever an opponent casts a spell, if no mana was spent to cast it, counter that spell."
Ice Cave86%
$0.77↑#21126↓0.9225.3 years↑
"Whenever a player casts a spell, any other player may pay that spell's mana cost. If a player does, counter the spell."
Planar Chaos80%
$2.31↑#7147↑1.41↑24.3 years↑
"Whenever a player casts a spell, that player flips a coin. If they lose the flip, counter that spell."
Chalice of the Void78%
$16.39↑#9268↑1.14↑22.9 years↑
"Whenever a player casts a spell with mana value equal to the number of charge counters on this artifact, counter that spell."
Nix76%
$0.97↑#25501↓0.24↓19.3 years↑
"Counter target spell if no mana was spent to cast it."
Nether Void75%
$519.01↑#161712.23↑32.3 years↑
"Whenever a player casts a spell, counter it unless that player pays 
."
."
Presence of the Master74%
$0.46↑#26110↓1.27↑32.3 years↑
"Whenever a player casts an enchantment spell, counter it."
Roiling Vortex72%
$1.96↑#2476↑0.45↓5.9 years
"Whenever a player casts a spell, if no mana was spent to cast that spell, this enchantment deals 5 damage to that player."
Hesitation72%
$0.30↓#22967↓0.13↓28.5 years↑
"When a player casts a spell, sacrifice this enchantment and counter that spell."
Bazaar of Wonders72%
$2.52↑#28239↓0.50↓29.9 years↑
"Whenever a player casts a spell, counter it if a card with the same name is in a graveyard or a nontoken permanent with the same name is on the battlefield."
Chancellor of the Annex71%
$2.86↑#10169↑0.80↓15.3 years↑
"Whenever an opponent casts a spell, counter it unless that player pays 
."
."
Lunar Force70%
$0.25↓#21116↓0.22↓10.1 years↑
"When an opponent casts a spell, sacrifice this enchantment and counter that spell."
Counterbalance70%
$2.58↑#3290↑1.0120.1 years↑
"Whenever an opponent casts a spell, you may reveal the top card of your library. If you do, counter that spell if it has the same mana value as the revealed card."