"Colorist" is a pretty broad term. I suppose in its broadest sense it could include anyone who is not painting monochromatically. Often those painters who use paint directly from the tube and avoid the duller earth colors like raw umber etc. are called colorist.
Many would say someone like Wolf Kahn is a colorist. However, I find his color a bit too arbituary and nonsensical.
I think of Henry Hensche as a colorist. In fact, he called himself a colorist as opposed to being a "value painter". He said every form change is a color change. In other words, form is created on the canvas not by lightening or darkening color by adding white or black, but instead by painting the distinct and unique color that desibes the form.
Some of his former students have set up a website in his honor if you would like to investigate him further.
http://www.thehenschefoundation.org/
Steve