Cerulean:
deep blue in color [Cambridge Dictionary]
Noted in “These bizarre lights in the sky hint at a way to predict earthquakes,” Nathaniel Scharping, NewScientist (6 May 2023, paywall):
Along with trembling buildings and shaking trees, those caught in the quake also witnessed something substantially more eerie. A barrage of blue lights, like flashes of cerulean lightning, lit up the night sky, apparently right above the fault line. This strange display was an example of what are known as “earthquake lights”, a semi-mythical phenomenon that has cropped up in reports of tremors for centuries.
