Many of my students ask: what is the difference between the Old Master’s paintings and those that were done during the nineteenth century by the Impressionist, didn’t they both work in oils? The difference is not whether they used oils, watercolor or gouache: it lies in the methodology they used in applying the pigments combined with where they worked.
The Old Masters worked in their studios and most often in a method referred to as grisaille (gray tones) to establish values first and then they added color glazes over the values. The Impressionists worked in plein aire (outdoors/in the streets) in a method known as Alla Prima, a method of working directly on the support (canvas or board) with a loaded brush of full pigment. The location of where they worked affected their subject matter, which is a whole different story.
Today, artists often work with both methods and in both places by sketching in plein aire and taking a photograph home to finish the work in the studio. This system was advanced also by an Impressionist, Gustave Caillebotte.