Why does the drain hole in the sink at home change color?

This is a conversation between a buyer and an engineer
Q: We have installed new tiles and a new base sink, giving our bathroom a new look. Less than a year later, the sink near the drain hole began to discolor. The old washbasin had the same problem, so we replaced it. Why does the sink change color and the toilet not? Sinks are purchased in large stores, while toilets come from different manufacturers – purchased in pipeline stores. Does it matter? Our other sinks, bathtubs, or toilets will not experience discoloration issues. We have well water and hard water, but we have water filtration and softening systems. I have tried using regular cleaning agents, such as vinegar and baking soda, but they have not helped remove stains. The sink still looks very dirty. What can we do?

A: This appears to be a problem with the supply line leading to the faucet. It looks like the water in your house comes out of the filter without iron, but then it has to go through a maze of probably old and new pipes to reach the various appliances. Since it stained the old sink and nothing else, now the replacement sink is painted but still showing nothing damage, the culprit is probably the connection to that sink. Try testing the tap water in your bath and comparing it to water from another appliance. This may help find the cause of the problem.