The attack says discard a {R}{W}{L} energy attached to Lugia, if I had a Rainbow or Multi Energy on top of the 3 basic energy to pay for the attack, could I just discard the rainbow/multi energy and keep the other three on there in order to attack?
"Rainbow Energy provides every type of energy but provides only 1 Energy at a time."
Elemental Blast asks you to discard 3 energy.
The answer should be pretty self-explanatory: no you can't. (Would it make sense to lose only 1 energy for a 200 damage attack?)