First, we have studied how color appearance depends on the ambient illumination as established by the background color. The color appearance changes caused by changes in the background mainly can be explained by assuming that the signals from the three cone classes are scaled by a factor that depends on the background color. This classic notion, called von Kries adaptation, has had a long and controversial history. In our experimental measurements of color appearance on CRT display devices, simple variants of this model predict performance quite well. although there are some small and systematic deviations This general principle of receptor scaling has gained enough acceptance that it is being proposed as an important component in modern color appearance models that are being considered for international standards