atla-annotated:

Azula’s Fire was not Always Blue

What that says about what her mom leaving and Zuko getting banished has done to her i.e. her bending, like everyone else’s in the Fire Nation, is fueled by rage. To turn that blue she must have an incredible amount of hurt/rage bottled up inside. 

Edited: Iroh’s explanation of ‘how you need inner balance for lightening’ is proven wrong in the show itself. If it were true Azula would not be able to throw lightning in the finale.