24 minutes ago, drpaladin said:

Modern cerulean blue is a darker and greener color than it was originally.

Cerulean - Wikipedia

Pigment characteristics

The primary chemical constituent of the pigment is cobalt(II) stannate (Co
₂SnO
₄). The pigment is a greenish-blue color. In watercolor, it has a slight chalkiness. When used in oil paint, it loses this quality.

Today, cobalt chromate is sometimes marketed under the cerulean blue name but is darker and greener^[a] than the cobalt stannate version. The chromate makes excellent turquoise colors and is identified by Rex Art and some other manufacturers as "cobalt turquoise".

Cerulean is inert with good light resistance, and it exhibits a high degree of stability in both watercolor and acrylic paint.

The word cerulean means sky-blue or dark blue, coming from the Latin word caeruleus (meaning blue, dark blue, or blue-green.  The specific artist's pigment "cerulean blue" was introduced later in the 19th century (around 1805 by Albrecht Höpfner, commercially popularized by the 1880s).  

According to the Oxford English Diectionary (OED), the earliest recorded use of cerulean as an adjective dates to the late 1600s (1590s), with the earliest documented evidence of the synthetic pigment cerulean blue (cobalt stannate) used in English appearing in 1677 by Thomas Herbert.  The first recorded use of cerulean as a noun dates to 1756.  In 1860 cerulean was commercialized and popularized for artists under the trade name coeruleum by English colorman George Rowney and in 1885 the specific term artist's cerulean blue entered regular usage.  The  variant ceruleous appeared earlier as an adjective in the 1570s. 

One of my favorite color words, right next to chartreuse.