When I first got into geckos I had to ask what being "fired up" meant. It is a change in color that reflects a darker pigment. It is in reaction to the environment. I have discovered they are more likely to fire up if you take them to dark moist to bright light. I have had them fire up from simply taking them out of their enclosure. They fire to differing degrees and can even partially fire. Bellow is an example of Phin unfired and then firing. He actually will fire much darker but you get the drift.