Color does not really exist. What we see as color is the result of different wavelengths of light being reflected off photoreceptive cells in our eyes. The rods reflect light in low levels of illumination. These are used more at night. The cones reflect light in brighter levels of illumination and are used during the day. White is a combination of all colors, and black is the absence of color.
Basic color theory, based on Sir Isaac Newton's development of the color wheel in 1666, says that the primary colors, colors that cannot be created by mixing other colors, are red, blue and yellow. These are the colors that all other colors are derived from. Secondary colors are created by mixing primary colors; red and blue make purple; blue and yellow make green and red and yellow make orange. Mixing a primary and secondary color produces a tertiary color and is given a two word name such as blue-green. The term "hue" refers to the color itself, and "value" refers to the level of lightness or darkness of the hue. Adding white to lighten a color is called "tinting." Adding black to darken a color is called "shading."
Anything that involves designing an appearance involves color, whatever the medium. Architects use color. Fashion designers use color. Graphic designers use color. Chefs use color. They manipulate and mix colors to create a look that conveys a message, a character, perhaps, that they have in mind. People use color theory to decorate their homes without ever having studied color theory. Indeed, it does not take knowing about primaries or secondaries to appreciate the effects of color, but an understanding helps.
Different combinations of color produce different effects. A monochromatic color scheme, for example, uses different shades of the same hue, and is not as bright as a color scheme that uses different colors. A boy's room might have striped dark blue curtains and bed spread, a lighter blue on the walls, and a different shade on the carpet. If you want brightness, try an analogous color scheme using colors that are next to each other on the color wheel. Complementary colors are opposites on the color wheel and make each other pop. An example is the use of red and green for Christmas.
The significance of color theory lies in the ability of color to affect emotion. Reds, oranges and yellows are considered warm colors because they advance. They pop. A fire, for example, is the first thing to draw the eye when you enter a room. They are exciting, but they can also be threatening. Fast food restaurants use warm colors such as orange on their walls because it disturbs. They don't want people lingering. Cool colors, such as blues and greens, recede. They are more relaxing, such as the blue of the ocean or the green of a forest. Colors that clash, such as pinks and oranges, can be deliberately used together to confuse and shock. An abundance of dark, such as black, is usually associated with death, or in clothing, sophistication. For all of these ways, the purpose is to communicate, to affect and to be noticed.