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I was trying some sort of Oath Tron, which would let me play planeswalkers of any color with colorless mana. However when I'm trying to cast Garruk, Apex Predator, the game asks me to pay 7 colorless mana, however won't let me pay anymore with 2 colorless remaining, which is the black and the green mana. Same issue with Nicol Bolas, God-Pharaoh, it'll get stuck with 3 colorless, requiring me to pay the GBR with colored sources.