Impressionism - Ultramarine Blue

Bernhard Gutmann, Breton Fishing Boats, 1912.
Childe Hassam, The Silver Veil and the Golden Gate.
Claude Monet, Rocks at Belle Isle Port Domois.

Though it sometimes seems as though the works of the Impressionist masters have been imitated to death, there is much to learn from them.
The emphasis on capturing effects of light and atmosphere, the unification of the image through colour and pattern, the vibrant optical colour mixing, the energy of the brushstrokes - these things will always be relevant to painting.
As seen in these works, the Impressionists celebrated Ultramarine Blue. They used it for skies, seas, and shadows (unmixed with white, it can be used almost as a black). Ultramarine was originally made from ground semi-precious stone (lapis lazuli) imported from far off central Asia. It was prized for its ability to simulate the azure of skies, but it was prohibitively expensive.
In the 19th century, chemists discovered how to make a much cheaper artificial version of the pigment with almost the same beauty and qualities.
The Impressionists also made use of Cobalt Blue and Cerulean Blue (both also first produced in the 1800s). Ultramarine is a favourite mixing blue, as it doesn't overpower the mix.