A paper (or other material) will absorb light of only certain wavelengths, depending on its chemical composition, and reflect the remaining wavelengths back toward the observer. These reflected wavelengths are a paper's color. Dyes or other fillers added to white paper are the chief determinants of a paper's color. The human eye alone is insufficient for the proper determination of a color value of a particular paper, and a color value will also vary based on the type of illumination used, so carefully cut paper samples are put in color-matching booths, where TAPPI-prescribed levels of illumination (i.e., simulated daylight) are shined on a paper at right angles to its surface. It is important in color matching that the paper samples themselves be chosen with care; the paper samples should all have the same grain direction, and the same side (either felt side or wire side) should be illuminated. A spectrophotometer is an instrument that analyzes the reflected light from a paper surface wavelength by wavelength, and generates a spectral reflectance curve, or a graph of the degree of light reflectivity from a paper's surface against each individual wavelength along the visible spectrum.
In the human eye and brain, color is a subjective quality. An objective measurement, in addition to spectrophotometry and specular reflectance curves, involves the use of color-matching scales, which plot the degree of reflectance of the three primary colors—red, green, and blue—on a three-axis graph. These "tristimulus values" are used to define a color. (See CIE L*a*b* scales. Other scales use degree of blackness or whiteness, a degree of redness or greenness, and a degree of yellowness or blueness (in other words, color oppositions). (See also Hunter L,a,b values.)