The colors you see are usually not monochromatic, but rather are made of a distribution of visible wavelengths, characterized by an intensity-distribution curve.

Colored lights appear the same to you, no matter what their intensity-distribution curves, provided that three of their properties are the same: their hue (main color or dominant wavelength), saturation (purity), and brightness (apparent overall intensity). For surface colors, the third property is lightness, (whiteness, grayness, or blackness). All colors can be arranged according to these three properties in a color tree. Two colors that look alike but have different intensity- distribution curves are metamers.

Overlapping spots of light combine their colors by additive mixture. Any color can be matched by an additive mixture of three colors, if we allow negative amounts. Colors that additively combine to produce white are complementary colors. The results of additively mixing colors depend only on the colors' chromaticities - the rules are represented by straight lines on a chromaticity diagram; for example, complementary colors lie on opposite ends of straight lines that pass through the point representing white. The greatest variety of colors can be obtained by an additive mixture of blue, green, and red, the additive primaries. In addition to simple addition, produced by overlapping illuminations, additive mixtures can be obtained by partitive mixing (small, closely spaced spots of different colors), and by rapid, successive presentation of different colors at the same place.

The subtractive process occurs when certain wavelengths are removed more than others from a beam of light, say when it passes through a filter or when it reflects from a painted surface. The results obtained when white light passes through two filters successively are found by multiplying together the transmittance curves of the two filters. (For reflection, the reflectance curve of the surface plays the same role.)

Subtractive mixtures can results in surprising colors. Colors formed by the subtractive process depend on the intensity- distribution curve of the illuminating light as well. Light sources are often characterized by their color temperature - the temperature of a matching incandescent light. The quality of a white light source depends on how white it looks, its ability to render color in a desirable fashion, and its efficancy (how efficiently electric power is converted into light).

Water colors and printer's inks are fairly transparent, but their mixtures involve both subtractive processes (similar to filters) and additive (partitive) processes. Printers make good colors by four-color printing, a process that uses inks of the three subtractive primaries (yellow, magenta, and cyan) plus black. Different colors are mixed by printing them in halftones - arrays of fine dots in each color, the size of the dots proportional to the desired amount of that color. Mixing colors with pigments and paints has many complications due to the heterogeneous nature of paints. The color of a paint depends on such details as the pigment size, the relative index of refraction of pigments and vehicles, the amount and nature of the absorption, reflection, and transmission of the various pigments added, whether these processes are selective or nonselective, even on the nature of the surface reflections.

That all colors have three attributes demands that there be three cone types in the normal color observer. Based on the information of

• spectral complementaries
• hue discrimination
• microspectrophotometry

Though there are only 3 different cone types, the perception of color is related to the four psychological primaries to describe the appearance of any hue: blue, green, yellow, and red.