Dark coloured bottles interrupt the path of light such that light cannot reach silver chloride in the bottles and its decomposition is prevented. It is known that silver chloride decomposes to silver and chlorine in the presence of light.Silver chloride on exposure to sunlight may decompose in the pattern given below:2AgCl → 2Ag + Cl2Therefore, it is stored in dark coloured bottles.

Silver chloride(AgCl) get decomposed in the presence of sunlight or light and form Ag & Cl2 therefore it is advised to keep AgCl and also AgBr in dark coloured bottles.
Silver and cl gas by decomposition
Because agcl react with sunlight and form silver