Up until the 15th century, the most common medium for painting was tempura paint. Tempura paint dries very quickly, meaning artist using tempura could not easily blend colors. Tempura paintings have been found from about 3000 BC. on early Egyptian sarcophagi decorations, thus showing how long tempura had been the most common medium. This shows monumental the transition from tempura to oil paint truly was. Oil paint began to make its way to Europe in the 12th century but was not widely adopted until the 15th century. The appeal to oil paint was that it dried slowly which allowed artist to continually work the paint. This allowed for artist to blend the paints thus allowing for softened transitions between colors, and results in a more accurate depiction of shading. The technique spread to Italy in the late 1400's starting in Venice and by 1540 oil painting completely dominated as the most popular medium - tempura was no longer being used at all. Oil paints often only came in a few premade colors, there artists had to mix small amounts of colors to create more shades, often done on a wooden palette. The crescent shaped wooden palette that is typically a symbol of an art, is in reference to oil painters. 

Sources: https://en.wikipedia.org/wiki/Tempera    

 https://en.wikipedia.org/wiki/Oil_paint

 https://en.wikipedia.org/wiki/Oil_painting

Image retrieved from Wikimedia Commons:  https://commons.wikimedia.org/wiki/File:Cadmium-paints-amy-lavine.jpg