Two or three colors on a plate are usually more interesting than just one. Visualize this combination: poached chicken breast with suprême sauce, mashed potatoes, and steamed cauliflower. Appetizing? Or how about fried chicken, French fries, and corn? Not quite as bad, but still a little monotonous.
Many hot foods, especially meats, poultry, and fish, have little color other than shades of brown, gold, or white. It helps to select vegetables or accompaniments that add color interest—one reason why green vegetables are so popular.