Colorimetric measurements are equally influenced by the reflectance spectrum of the object and the illumination spectrum of the light. The 1931 CIE colorimetric measurements are made one pixel at a time; they integrate the radiances at each wavelength with three colormatching functions so as to generate three Tristimulus Values for one pixel. No information from other pixels in the field of view is used in this calculation.Our everyday experience is that color appearance of objects remain the same, regardless of substantial changes in the spectrum of the illuminant. In other words, everyday experience tells us that an object's reflectance spectrum controls appearance, while its illumination spectrum has little influence.This paper will review the history of different hypotheses explaining human color constancy and describe techniques for measuring color appearances. It will review important experiments that measure color sensations and new techniques using the introduction of a new patch in a display that destroys color matches.Human color vision is a field phenomenon. Humans calculate color sensations by comparing pixels across the entire field of view. Global changes in reflectance or illumination cause small changes in appearance: Local changes in reflectance or illumination cause large changes in sensation. The spatial interaction of all pixels in the field of view controls human color appearance.