I was just curious, so I checked on Google Trends to compare relative search volumes for different painting media.
People search least often for the word "gouache,"(in blue) along the bottom. "Oil paint" (in yellow) and "acrylic paint" (in green) are two or three times more popular than gouache. Acrylic passed up oil in popularity about three years ago.
Interest in "Plein air" (in
blue purple) appears to be very seasonal, spiking in the warm months.
"Watercolor" far surpasses the others. It sagged a few years ago, but it's rising steadily. The search volume may not translate directly to the popularity of the art medium. The spike at the red letter "K" is tied to a popular article about digital printing watercolors on fabric. The letter "B" aligns with a story about a photo-to-watercolor app.
Casein painting didn't register at all—which is one of the reasons I love it.
Edit--Here's a graph of the usage of those terms as printed in books in the English language courtesy the Google Ngram Viewer. (thanks, Piers!) The graph may be a little misleading because it only tracks the frequency of the use of those particular words or phrases. Oil is just assumed most of the time people write about painting.