Today, designers use color wheels, models, and theories to determine which colors coordinate or clash. So, what is color theory, and how does it work? Essentially, it's a guideline for color mixing, allowing visual artists to select the most effective colors for their projects. Renaissance men like Leonardo da Vinci and Leon Battista Alberti were the first to decode color in this fashion, but it would be a few more centuries before color theory was an accepted field.
The Color Wheel
First developed in the 18th century, the color wheel is a tool for understanding how colors relate to each other. It places the primary colors at equidistant points around a circle, with the secondary and tertiary colors in between. Using this tool, you can determine which colors produce the most compelling combinations. For example, complementary colors are found on opposite sides of the wheel (i.e. green and red), while analogous colors are adjacent to each other on the wheel (i.e. red and orange).
One way to create new colors is by mixing them together. This is the basic premise behind the additive color system, where the primary colors (red, green, and blue) serve as a starting point for creating any other color in the spectrum. When you combine two of the primary colors together in equal measure, you create the secondary colors (cyan, magenta, and yellow).
On the other hand, subtractive color starts with the reflected light spectrum, so when you add the three primary colors (in this case, cyan, magenta, and yellow) together, you get closer to darkness. In this system, black is the combination of color, and it prevents light from being reflected. However, since it's impossible for color pigments to fully absorb light, we also add "key" (the "K" in CMYK) to this color mixture. Key is basically black, and it's the final push that takes your CMY mixture from a weak brown to a deep black.
This attribute measures how bright a color is, starting with black (0% brightness). A red at 100% brightness, however, will be the most vibrant red color. This is not to be confused with "lightness", another attribute that measures the variation between a color's full range of brightness.
This attribute measures a color's richness. With a high saturation, colors are much more intense and vivid, while a low saturation eliminates color from an image. At 0% saturation, the image lacks color entirely and can be classified as "grayscale".
Finally, this attribute tells artists where a color is located on the color wheel. For example, green is found at 120º, while yellow is found around 50º. We use degrees to measure hue because it's the easiest way to determine a position on a circle.