Hi! Will answer your question in two parts. First a gap in the teeth can happen for different reasons. If you have noticed the gap increasing then you need to get a xray done so your dentist can check if your bone is getting damaged which very often leads to loosening of teeth. Immediate corrective action needs to be taken to prevent any further bone loss. Once the basic problem is taken care of the yellow colour of the teeth can be easily corrected by tooth whitening which is a simple procedure.. I would advise you to see a good dentist at the earliest without any more delay.