Color sensations usually make their impact on the unconscious mind. Consequently people are often unaware of the tremendous influence colors have upon them. A person may be unconscious of the colors around him and yet be powerfully affected by them in regard to mood, temperament, and behavior.

Color sensations can produce physical reactions. People often feel cold in a blue room and warm in a red room without realizing that colors, not physical temperature, are responsible for the difference in effects. Colors derived from blue—the "cool" colors—have a sedative effect and have proved calming to highly nervous people. Less excitable people may become depressed in surroundings of strong blue and have their spirits lifted by "warm" colors—those derived from reds and yellows. Most people have a strong unconscious tendency to seek a balance of calming and stimulating colors by surrounding themselves with tints and tones derived from both cool and warm hues.

Our preference for a color depends not only on its tonal value, but also on how well it harmonizes with other colors around it. The amount of surface covered by a color also affects our feeling about it. A bright red wall might be disturbing, but the same color, if limited to small areas, can be quite pleasing. Certain colors are associated with certain objects, and the "wrong" color can be startling, unpleasant, or offensive. For example, green egg yolks look unappetizing because yellow is normally associated with yolks. Careful attention must be paid to the painting and lighting of dining rooms to prevent good food from being "spoiled" by the color of the room.

Since color affects people in definite ways, wise use of color in interior decorating is important. The color of a room should be chosen for its psychological effect—the mood it creates—and for its harmony with objects in the room. Color also has an important effect on illumination. Rooms that receive little sunlight should have light-colored walls and ceilings, which reflect a high percentage of light.

Color is widely used to attract attention, as in signs, advertisements, or danger signals. Fire-fighting equipment is red or yellow, and stop lights are red. The highly visible combination of black and orange is used to indicate hazards or mark road signs. Green is the color of safety; a green traffic light means it is safe to proceed, and a green cross indicates first-aid equipment. In nature, the brightly colored plumage of male birds attracts mates.

Color is also used in the opposite way—to make objects blend with their surroundings, as in camouflage or protective coloration of animals.

Color can be used to identify objects—such as wires in telephone or television circuits—or to identify people, such as the players on a football team. Color is also important in science and industry. For example, certain colorless materials admit ultraviolet radiations; color is used to keep such radiations out where they would be injurious. Glass containers are often colored brown or dark green to protect drugs and foods from harmful rays.

Many colors have symbolic meanings. For example, red can represent (among other things) festivity, anger, or danger. The various meanings are by no means universal; black, for example, is the color of mourning in Western countries but in parts of Asia white has that association. The vestments and hangings in some churches are colored according to the season of the church year. For example, purple, representing penance, is used during Advent.