The color Cerulean, a light sky blue, is traditionally made from the PB35 or PB36 pigments. It’s a semi-opaque, granulating, green-toned blue with limited range of values, erring on the side of being light-colored. It’s nonstaining and highly liftable, making it a good choice for skies (if you like granulating skies). Personally, I usually like a less textured sky – but Cerulean has other uses as well, such as being a beautiful textured green mixer, and muting earth tones into cool, granulating browns.
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 or any other brand), made from Phthalo Blue + white. PB15 is not the same color, and will not have the same granulation/magic/mixing properties.