DID YOU KNOW? Mega Charizard X's flame are much more hotter hence, higher temperature intensity. We'll be tackling it below.
The flame's coloration solely depends on the amount of chemical burned in the atmosphere (oxygen).
Cooler flames are yellow (seen in Charizard), while hottest flames are blue & white (Mega Charizard X).
Temperature of flames CAN be controlled by the amount of present oxygen.
With smaller amounts of oxygen, only it burns yellow.
With higher amounts of oxygen, it burns blue & white.
Simply to say, when Charizard evolved into Mega Charizard X, Mega Charizard X's oxygen saturation level is increased by tenfold.