- Can I use Umbreon-GX's "Dark Call GX" attack to discard 2 Double Colorless Energy?
- Yes, you can select 1 Energy of each Double Colorless Energy to discard both of them. This applies for discarding your own Energy for an attack and for retreating as well. So if you used Hoopa-EX's "Hyperspace Fury" attack or paid its Retreat Cost of 2, discarding 2 Double Colorless Energy would be legal options.
Source:
TPCi Rules Team (2017-01-26), TPCi Rules Team (2017-06-27)