Cerulean, meaning “sky”, is a light sky blue, traditionally made from the PB35 or PB36 pigments, which are both cobalt oxides. It’s an opaque, granulating, light-valued, green-toned blue.
Warning: Be careful to look at pigment numbers. Some brands, like Mission Gold, call their PB15 Phthalo Blue “Cerulean.” Don’t make the mistake I did when I first started painting, and get “Cerulean Hue” (from Da Vinci, Cotman, etc.), made from Phthalo Blue + white. PB15 is not the same color, and will not have the same granulation/magic/mixing properties.