If I have two Rainbow energy attached to a Dark Charizard and I flip two heads, can I discard the Rainbow Energy or do I have to have Fire Energy cards?
You could discard the Rainbow Energy as they do count as all basic energy types.
Source: WotC Chat (2000-06-22)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS1_104
HGSS4_7
PL1_121