If I have a Dark Energy, a Rainbow Energy, and a non-Fairy energy card attached to Zygarde, when I use Zygarde's "Core Enforcer" attack can I discard the Rainbow Energy first for the Darkness discard, and then without a Fairy energy attached I'm done?
No, since you have Rainbow satisfying the Fairy Energy condition you would have to discard both the Rainbow and the Dark Energy.
Source: TPCi Rules Team (2017-09-21)

Relevant cards

Note: All applicable cards may not be listed below.
SM3_100