Firstly you need to differentiate between yellow teeth or yellow deposits. If it's yellow deposits, which looks like extra on teeth please undergo scaling. When these deposits are present it can also cause bad smell so go for scaling.
And if you are getting your scaling done evey six months n still it's yellow teeth then the reason for teeth staining could be many. So consult dentist. And we have a lot of options to treat yellow teeth and it's case specific.... And yellow teeth as such doesn't cause bad smell. So I assume you have deposits/ calculus issues than yellow teeth itself ❄❄❄
Yellowish discoloration is generally due to the formation of a layer of tartar on the tooth structure. This can be easily eliminated with professional cleaning. Bad smell might be due to numerous reasons, the most common being tooth decay and gum inflammation. Again professional cleaning along with cavity fillings will eliminate ur problem
We can suggest you a generalised treatment options as of now ...because the specific treatment for the problem you raised can be rendered only if you can consult in person.
BTW the reasons of yellow teeth can be various eg stains due to consumption of stained food and habits of an individual
Those can be extrinsic or intrinsic
Extrinsic stains can be removed by simple scaling or clean up
Intrinsic stains can be removed through various procedures like bleaching to smile designing.
And you concern about bad odour ....again this depends on your oral hygiene and the way it's maintained.even your routine intake of food and systemic or general health also needs to be considered for the above asked query.
Mostly it could be due to dryness of mouth so consume plenty of water and dun stay without having water for longer duration
Get a consultation from your dentist ASAP.