A colorist is someone who understands colors and checks the color balance of the scenes in the film or TV show. They change the color saturation ad other camera techniques to get the right color shade for the scenes to match up accurately. He/ she work towards getting the right color sequence as required by the script and color the scenes accordingly.
The pictures or videos might not be taken accurately and need to be edited by correcting its color. They are the ones who give artistic advice for the entire film visually. Colorists are also responsible for recording and saving every color sequence for future purposes.