Question

Taj Mahal has become yellow in colour

Answer 1

Due to corrosion and exposure to gases like methane,carbon dioxide which produced from the nearby factory when the gases comes in contact with air ,water etc which decolour the white marble

Answer 2

It is because due to the acid rain. When acid reacts with marble it forms a yellow coloured precipitate.
H2SO4 + CaCO3 = CaSO4 (yellow ) + H2O + CO2.