Physics observables for color transparency

The physics observables dedicated to the study of color transparency are diverse. After a brief pedagogical introduction, we emphasize the complementarity of the nuclear filtering and color transparency concepts. The importance of quantum interferences leads to suspect pictures based on the preparation of a squeezed state of small transverse size leading to a rapid transverse expansion. The different roles of heavy and light nuclei are emphasized. The possibility of color transparency tests in the vacuum is stressed. Such a test may be quickly addressed in the virtual Compton scattering reactions at CEBAF or HERMES energies.