No one can tell exact cause without looking at tha case There can be various number of reason for such problem If you cam visit clinic , i can tell you the exact cause and exact treatment

There are many causes of yellow teeth like fluorosis stains,pan masala or gutkha stains, tea or coffee stains, stains due to some drug reactions etc. But in your case as the tooth is some part yellow and some light yellow then it may be also be caused due to some trauma to teeth while eruption or after eruption. Exact cause and treatment part can be decided only after clinical examination. But according to your description of the problem the best treatment option is covering the tooth with metal ceramic or zirconia cap.  For any more suggestion consult me on docs.animesh @gmail.com . Thanks

cause of yellow teeth may be many. it may be because of stain ,calculus or may be congenital problem or upper tooth layer not formed completely but the nice thing is that is for problem treatmemt is available. you have to visited dentist for proper check up and diagnosis of your disease then treatment will be decided