Art directors/production designers make sure that the colors being used in the scene are appropriate for the mood and the story they are trying to tell. Everything from costuming to set pieces and props falls under the Art Director. The reason it always looks flat is because that's how digital is shot in order to preserve the most detail in the image- depending on what you are shooting it is often referred to as log, or raw. The colorist colors the image, but typically won't make changes to like the color of someone's clothing, changing a shirt from red to blue- because the art director (or costume designer under the art director if you have a large enough crew) picked out that specific color for a reason. Hope this helps!