"For centuries, color---and the pigments to create it---was as precious as gold. Imagine if I, as a color expert, held the secret formula to the perfect sky blue paint and, to my colleagues' chagrin, only my clients could have access to that hue for their walls.
This kind of scarcity and level of secrecy was once common in the world of color. Artists would go to great lengths to protect and keep secret their color formulations. Costly pigments were held under lock and key. Even the most prominent artists relied on wealthy patrons to front them the money to procure expensive pigments. For example, if Vermeer had a client who commissioned a painting containing a certain amount of costly ultramarine blue (lapis lazuli, ground as pigment) that patron would have to advance the money so the artist could buy the precise amount of the pigment needed. And, Vermeer would have to accept visits from financial auditors who would make sure that the pigment was being used as contracted and that none was being squirreled away by the artist or his helpers."
Vermeer's 'Girl with the Pearl Earring,' also called 'The Mona Lisa of the North' pictured above included the vibrant ultamarine blue in her face-framing turban or scarf. The color sets off her skin tones very beautifully.
Today we artists are privileged to have at our fingertips almost any color at a relatively affordable cost. I will never gripe about the cost of paint or art supplies again.
On another note, just wanted you to know the difference in angels depicted in old paintings.
Cherubs are those portrayed as winged infants with chubby, rosy faces. They are depicted in western art from the 15th century onwards. Putto, "little boy" in Italian, is the chubby, naked child represented in art since classical times, often as a decorative feature. Often, putti are adorable, cherubic heads with wings and no bodies.
Originally derived from ancient representations of Eros, the Greek god of love, since the Renaissance putti have often been associated with his Roman counterpart, Cupid. Cupid is normally represented as a naked, winged boy with bow and arrows.
From: The Concise Oxford Dictionary of Art Terms.