The fire started red. No reason for it to start otherwise; Lin's fire was always red at the beginning.
The kicks fed it, each impact adding something to the cycle that made the flame consume more energy in less time and release more than it received, and the red shifted toward orange first and then, when the accumulated impacts crossed the threshold, toward blue, the fire that Lin produced when she had the right conditions to produce it.
The two beasts couldn't attack because attacking required moving and Lin's insane kicks didn't allow movement.
Their attempts to press together and trap her in the middle only added to the impact because Lin converted their movement into part of her own.
The group was watching from the first step of the descent.
