As a couple of people have mentioned, if it is age related discolouring, then whitening at dentist is the solution.The yellower core of the teeth show thru enamel as it thins / ages, and also worsening the effect of surface staining.
Local dentist it’s £360 – this covers a custom mould top and bottom and sufficient treatment gel for the initial whitening, and recommended left over gel is used for a one off top up every 6 months (so ~ 18 months)
After that, it’s £50 for further gel that covers 4 top ups so ~ 2yrs
So expensive initially, but I’d say fairly reasonable to keep it in that condition.