First of all if the yellow color change is more generalized over your skin, then this is something called jaundice, which is a serious symptom which needs to be investigated right away, as it is often a sign of a problem with the liver. On the other hand, if the yellow color is just limited to right around your navel, then it is unlikely to be caused by a serious underlying medical problem.
I recommend that you see your primary care doctor
about this issue. They will be able to take a look at the area of skin that is affected and help you figure out what is going on.
One possible cause of dry skin around the navel is eczema
, which is a condition in which the skin becomes irritated and inflamed. Usually this just causes redness or itching, but if there is some thickening or flaking of the skin, it might look yellow, which could explain your symptoms.
Another possibility would be a fungal infection of the skin, which could result in some color change, dryness, and odor, potentially also with itching.
Finally the skin could be stained by some external object. For example, a navel piercing might do this, as might a belt buckle or metal snap on a pair of pants.
Go see your doctor at your earliest convenience for more help figuring this out.