Neitz, Carroll, Yamauchi, Neitz and Williams (2002) reported that color perception is mediated by a plastic neural mechanism that is adjustable in adults. It was demonstrated that long-term changes in chromatic experience can produce changes in color perception that can last for weeks. The long-term perceptual changes resemble traditional color aftereffects; for example, a light that appears unique yellow to the unadapted eye will transiently appear greenish after minutes of adaptation to a red light and, similarly, daily wearing of red contact lenses will produce a shift in the spectral locus of unique yellow. However, we propose that the long-term changes in color vision are fundamentally different from conventional chromatic adaptation in ways besides having a prolonged time course. Conventional chromatic adaptation represents an adjustment of the visual system that improves its ability to extract information about the spectral reflectance of objects as the chromaticity of illumination varies. The associated aftereffects represent the consequence of an inadequate temporal response in the adaptation mechanism; the system is slow in returning to neutral after prolonged exposure to intensely colored stimuli. In contrast, we propose that the long-term changes in color vision that can be built up by daily exposure to chromatic alteration are manifestations of a plastic neural mechanism that allows information from the environment to instructively reorganize neural connections throughout life. In this case, the associated aftereffects represent functionally appropriate adjustments in synaptic weighting of neural connections that will not return to their previous values unless instructed by experience to do so.