Doubt they'll even need the hair dye. These guys were able to destroy planets long before the transformation that makes them even more OP. Not much energy would be used up at all.

But they can be killed. In the anime, they almost always have someone to distract other enemies or support them in some way. If it were just Goku and Vegeta against the whole destiny universe, vex and wolves would be traveling through time, warsats would be falling on their heads, and you'd have legions upon legions of Gjallarhorn's firing at them. They wouldn't have time to think.

Legions and legions of Gjallarhorn cannot destroy a planet. Goku's Spirit Bomb attack can.

It's Goku v. Destiny universe, not DBZ universe. Goku can't get any energy.