The photochromic property and colloidal formation mechanism of the sodium chloride crystal material colored by γ ray irradiation and electron beam bombardment are investigated systematically. The F→C conversion can be created efficiently by irradiating the colored crystals with a few laser pulses at 448 K. The sizes of the produced colloids augment with the increase of the colored doses. A new colloid whose absorption band located at 625 nm is produced and observed for the first time in the colored sodium chloride crystal. The C→F conversion can be carried out efficiently at 532 K, so the reversible conversion from F center to C center is realized. The F→C conversion is realized mainly by means of the agglomerating of mobile defects produced by the photoionization of F and F' centers under the F band laser irradiation. The brief physical interpretation is given on the colloidal formation mechanism.