From the perspective of geomantic omen, some people think that the yellow tombstone means that the family fortune of the deceased is not good, while others think that the yellow tombstone is because the deceased worked hard and walked through many places before his death, so the tombstone absorbed the local atmosphere and could avoid some negative effects. But these claims have no scientific basis and cannot be confirmed.
If the family members are uneasy about the tombstone turning yellow, they can choose to brush or replace the tombstone to keep it clean and tidy.
In short, there is no clear answer to the question whether the tombstone turns yellow. In any case, it is only a symbolic expression, and it is more about expressing family members' nostalgia and respect for the deceased.