Well, yeah, you would have to if it wa the only energy... Actually, anything that discards a particular type of energy can be supplemented with a Rainbow-type energy.
Infernape requires a player to discard all Fire energy attached to it. So you could have 2 Fire energy, 1 Double Rainbow and 1 Holon's Castform attached, and you would have to discard all of them if you use Flame Blitz. You discard anything that provides Fire energy regardless of what energy card it is.
Abhorsen got to it before me...But Abhorsen is right, you discard every fire energy card attached to it, OR, whatever is PROVIDING the Fire energy, I.E. Rainbow, Double Rainbow, Scramble, Holons Castform.