White is a color too. Although it is achromatic, white can still be measured colorimetrically. With an ideal white, all the incident light in the 400-700 nanometer range is reflected. This "ideal white" is, however, purely theoretical. Barium sulphate, which was one widely used as a white standard, give an average reflection of only 98% across the visible spectrum.

"Whiteness" - that is, how white a material is - is a yardstick for judging the quality of many typically white materials such as paper or textiles. The "whiteness" of a material can come very close to an ideal white if the material is bleached, thus dstroying the color pigments that absorbed light. But is a white needs to be "whiter-than-white", optical brighteners may be used. These optical brighteners, which are found in everyday life in washing powders and toothpasts, for example, absorb radiation in the UV range (<400 nanometer), which is not visible to humans, and then release the energy as additional light in the part of the spectrum that is visible to human (between 400 to 700 nanometer). Materials that have been treated with an optical brightener can therefore return more than 100% of the light that would have been reflected from an ideal white surface. These materials appear to be "whiter-than-white".

In colorimetry, the "whiteness" of a material is usually indicated by a numerical value - the whiteness index. Many different whiteness equations have been used, including CIE whiteness index, the Berger whiteness and the Ganz-Griesser whiteness index. Whiteness evaluations of optically brightened materials depend directly on the amout of UV energy emitted from the light source of the measuring instrument.