When a solution of slaked lime is applied on the wall, CO2 gas present in the air slowly reacts with a thin layer of calcium hydroxide to form a thin layer of calcium carbonate, that is quite white. Therefore, the newly formed calcium carbonate impart a white look to the walls.  Ca(OH)2 (aq) + CO2 (g) →  CaCO3 (s) + H2O (l)