Cerulean and Indigo

Cerulean adjective - Deep blue like the color of a clear October sky.

Indigo and cerulean are semantically related in blueness topic. In some cases you can use "Indigo" instead the word "Cerulean" as an adjective or a noun, when it comes to topics like sky.

Indigo adjective - Having a color between blue and violet.

Cerulean and indigo are semantically related in colour topic. You can use "Cerulean" instead the word "Indigo" as an adjective or a noun, if it concerns topics such as blueness, sky.