Willow spread her wings and they burst into a giant flare, arcing over her body and wildly thrashing about. "I don't want to have to do this ro a poor tree, but I guess I have to." The fire was red, then yellow, then white, then blue, and then settled at a purple. Summoning fire this incredibly hot vastly ate up Willow's energy, but she just had to keep it up until she could make contact with the trunk
(Yes, really hot fire is purple, and if you get any hotter than that, the fire will become invisible. If you get to trillions of degrees, then you get radioactive fire.)