Diamond Mare 
Artifact Creature — Horse
$0.16 #7424 0.00 8.3 years
As this creature enters, choose a color.
Whenever you cast a spell of the chosen color, you gain 1 life.
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Cards similar to Diamond Mare 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.
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"Whenever you cast a spell, if it's at least one of the chosen colors, you gain 1 life for each of the chosen colors it is."
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$0.30↑#15130↓0.11↑7.3 years
"Whenever you cast a spell of the chosen color, put a +1/+1 counter on this creature."
+1 more matching line
Heraldic Banner84%
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"As this artifact enters, choose a color."
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Jeweled Torque82%
$0.24↑#30466↓0.25↑27.0 years↑
"Whenever a player casts a spell of the chosen color, you may pay . If you do, you gain 2 life."
. If you do, you gain 2 life."+1 more matching line
Gauntlet of Power80%
$3.80↑#2627↑0.20↑20.0 years↑
"As this artifact enters, choose a color."
Throne of Eldraine80%
$29.41↑#897↑0.28↑3.1 years↓
"As this card enters, choose a color."
Contemplation78%
$0.27↑#20942↓0.24↑28.6 years↑
"Whenever you cast a spell, you gain 1 life."
Setessan Battle Priest78%
$0.06↓#27390↓0.29↑13.1 years↑
"Whenever you cast a spell that targets this creature, you gain 2 life."
Paradise Plume78%
$0.19↑#11150↓0.0020.0 years↑
"Whenever a player casts a spell of the chosen color, you may gain 1 life."
+1 more matching line
Student of Ojutai76%
$0.05↓#18033↓0.24↑11.6 years↑
"Whenever you cast a noncreature spell, you gain 2 life."
Lifesmith76%
$0.18↑#30421↓0.0716.1 years↑
"Whenever you cast an artifact spell, you may pay 
. If you do, you gain 3 life."
. If you do, you gain 3 life."
Treva, the Renewer76%
$0.56↑#21707↓0.0526.0 years↑
"Whenever this card deals combat damage to a player, you may pay 
. If you do, choose a color, then you gain 1 life for each permanent of that color."
. If you do, choose a color, then you gain 1 life for each permanent of that color."
Dawnhart Geist75%
$0.14↓#17966↓0.064.9 years↓
"Whenever you cast an enchantment spell, you gain 2 life."
The Prismatic Piper74%
$0.09↓#28443↓0.20↑5.9 years↓
"If this card is your commander, choose a color before the game begins. this card is the chosen color."
Doomskar Oracle73%
$0.17↑#19561↓0.005.7 years↓
"Whenever you cast your second spell each turn, you gain 2 life."