Question

Why does white coloured silver chloride turns gray when kept in sunlight?

Answer 1

Explanation:
Silver chloride which is white colour turns to grey colour silver due to decomposition reaction which occur in presence of sunlight. Such reactions are called as Photo chemical reactions.
Here chlorine gas will escape.

Silver chloride (s)---> Silver(s) + Chlorine.(g)
 
           Sunlight 
2AgCl(s)-------------> 2Ag(s)+Cl2(g)
 white                       Grey 

Answer 2

In presence of sunlight have amount of light decomposes AgCl to silver and chloride ions.
little amount of actually they are black it spreads white so it looks gray.