Could yellow teeth be whitened if there is dental cavity?

I would get the cavity filled first, then whiten your teeth if you want to.  This is the best option.  

Thank you for your question.Cavities, especially if they are deep ones can leave the internal nerves of your tooth exposed.The ultimate cost of not treating cavities before doing tooth whitening is that you could end up causing yourself a completely avoidable root canal.See your dentist and get all your required treatment done first.Good luck and kind regardsThomas Dey, BDS, MSc

It is always better to get the cavity taken care of as the whitening could be very painful. That being said, you may need to redo the filling if it is in an area that is very noticeable.

Its best probably to restore the tooth first then whiten your teeth to avoid sensitivity / problems .
Following whitening depending on where the cavity is it may have to be re filled to match the new whiten shade of your teeth.