User blog comment:Lightning the Skrill/Night Fury's Plasma Blast; Blue or Violet?/@comment-24497928-20150324133733/@comment-7628730-20150403022102

Actually, there is no naturally occuring green flame. Green flames are only produced when the chemical structure of the flame is altered, such as exposing a flame to poisonous gas. Other ways to create green flames are to burn certain plants. This also applies to all other colors of flame that can plausibly  exist. However, blue flames, can be created either by burning certain substances, or by superheating a flame, like a welding torch, (you probably shouldn't look directly at a welding flame close up, it can blind you, although from at least, hmm, I would say, 150 feet away it would be safe). However, white flames are always going to be hottest, no matter what, and can't be created by buring certain substances, only through supering heating flames. It makes sense, then, that Toothless has blue or violet colored flames depending on the temperature that he fires them at.