Color Constancy has two hypothetical mechanisms: Chromatic Adaptation and Spatial Comparisons. These mechanisms have different fundamental properties. Adaptation models work with small individual scene segments. They combine radiance measurements of individual segments with the modeler’s selected parameters that scale the receptor’s cone quanta catches. Alternatively, spatial models use the radiance map of the entire field of view to calculate appearances of all image segments simultaneously. They achieve independence from spectral shifts in illumination by making spatial comparisons within each L, M, S color channel. These spatial comparisons respond to color crosstalk caused by the overlap of spectral sensitivities of cone visual pigments. L, M, and S cones respond to every visible wavelength. Crosstalk causes the spatial comparisons of cone responses to vary with changes in spectral illumination. Color Constancy works best in spatially uniform, and variable spectral illumination. Measurements of Color Constancy show systematic departures from perfect constancy. These limits of Color Constancy are predicted by spatial comparisons with cone Crosstalk. These limits do not correlate with Chromatic Adaptation models. This paper describes cone Crosstalk, and reviews a series of measurements of the limits of Color Constancy in variable spectral, spatial and real-life illuminations.