Okay, so I just found out when playing the BFZ PvE event earlier today, trying to use Karn on both World Breaker (the final boss of Part 1) and Inverter of Truth (the boss right before).
Against World Breaker, its abilty permanently fails to fire since Karn is indeed a colorless planeswalker, and thus he doesn’t really have gems of his color.
But against Inverter of Truth, its ability ingests everything except loyalty gems despite the fact that Karn is officially a colorless planeswalker who just so happens to be able to use all five colors of cards.
So maybe I’d like to ask also if you have encountered the same issue, especially playing Ulamog against Karn, since Ulamog’s ability is Ingest all gems of enemy colors, so does it happen the same way as Inverter’s case?
Just curious…