In presence of sunlight, the heavy amount of energy of light decomposes AgCl to silver (Ag+ ) and chloride (Cl-) ions. Now the little amount of silver flakes are actually black which is fully spreaded over white silver chloride which looks grey.
B'coz in presence of sunlight silver chloride decomposes in silver and chlorine gas is evolved. Silver chloride is white coloured while silver is grey in colour. The balanced chemical equation for above reaction is:
Surya Venkatakrishna answered this
When silver chloride is kept in sunlight it decomposes to form silver and chlorine gas. thus the white colored silver chloride turns into grye colored sillver. this type of decompositoin is photolysis or photodecomposition