One of the seven Wonders of the World, the 17th-century mausoleum flanks a garbage-strewn river and is often enveloped by dust and smog from belching smokestacks and vehicles in Agra.
Tiny insects from the drying Yamuna River into which the city pours its sewage crawl into the Taj Mahal, their excrement further staining the marble, an environmental lawyer recently told the Supreme Court.
The court slammed the government for not doing enough to preserve the monument, which was built by Mughal emperor Shah Jahan as a mausoleum for his wife Mumtaz Mahal.
"If the Indian scientists and the (conservationists) can't do the things, they should be able to contact foreign experts or conservationists, those who can come and they will be readily happy to help," said lawyer M.C. Mehta, who has been fighting to save the Taj Mahal from pollution for three decades.
Restorers have been using a paste of a clay mineral to clean the marble. It pulls away impurities from the surface and can then be washed off with water.
The change in colour has not come out of the blue. Environmentalists and historians have long warned about the risk of soot and fumes from factories and tanneries dulling the ivory monument.
Tourists visiting the monument said they hoped steps will be taken to preserve it.
"I think the Taj Mahal is one of the biggest icons of India and I think the city would be better to be cleaner and for the government to do something about this," said Francesco, a tourist from Argentina. "Because it is a shame, you know. Yeah!"