This issue could be due to a condition called Fluorosis. Excess fluoride level in water can cause yellow discoloration of teeth. It's caused by overexposure to fluoride during the first eight years of life. This is the time when most permanent teeth are being formed. Studies show that Theni district in Tamilnadu has high fluoride level in water. This fluoride will weaken the strength of the tooth.
But there are also other reasons for yellow discoloration of tooth, like excessive use of coffee and tea, poor dental hygiene, certain medicines, genetics, tobacco use etc.
In your case it would be mostly because of over fluoride content on the water. So, to solve this issue, you (your people) could reduce the fluoride content on the drinking water.