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.
Sparky Sparky Boom Man aka Combustion Man controls his bending via his ‘third eye’.
The third eye (also known as the inner eye) refers to a dharmic spiritual tradition (e.g. in Hinduism) where the third eye also known as ajna chakra. The anja chakra stands for 'command’ or 'summoning’. It is located around the middle of the forehead, slightly above the junction of the eyebrows. The third eye is often associated with religious visions, clairvoyance, the ability to observe chakras and auras, precognition, and out-of-body experiences.
Fun fact: Some animals (like bullfrogs) have a non-spiritual version of a third eye called parietal eye.
Image source: One