Fauve artists, such as henri matisse, extended the visual vocabulary of post-impressionism by making their primary subject matter for painting.

The word that best fills the given statement above would be "colour." Artists that uses the principles of post-impressionism like Henri Matisse usually uses the elements of colour as their primary subject in painting. The art movement began during the 1886 and 1905 which eventually lead to the rise of Fauvism.