Tonalist vs. Colorist
What do these two terms mean?
You may have heard your instructor talking about tone or tonalist when discussing painting. Tone refers to value (how dark or light something is). We use value to model forms. A tonalist then is one who picks colors based on their tone or value when modeling. Whereas as a colorist gives priority to hue. Hue change and color interaction are most important to a colorist. Yes, she often also considers the value of the color she adds, but color is foremost. And at times, she will model parts of a form without changing value, but just with changing hue. You may be surprised to hear that it can work.
In the visual example below, it is fair to say that an Impressionist is a colorist. Notice how the tonalist used the same red hue on all sides in differing values while the colorist used red, orange, and a slightly different orange to model her cube. You may even have seen that the red used by the colorist is cooler than the red used by the tonalist on the shadow side of the cube. The tonalist is not considering color temperature.
To learn more about how to use color to model form, take a look at the Cennini Method and Cangiantismo.