There are various reasons for this :
1.The levels of air pollution in Delhi are very high. Also the industries near taj mahal release toxic fumes and smoke into the air, polluting it even more. Since the air quality is very filthy therefore it has a very negative impact on the marbles of taj mahal.
2.The tiny insects which live in the yamuna river, which itself is very polluted due to the sewage waste which is being dumped in the river, crawl to the taj and their excrement further stains the marble.
3. Also taj mahal is situated beside a polluted river, enveloped by various industries and covered in dust all day long. Therefore all these factors are having a very negative impact on the marble quality of taj mahal and therefore it is changing its natural colour and turning to yellow.