Sclera colour, along with iris colour varies. Some people's sclera are whiter than others. This is a genetic matter that you can not do anything about.
If your sclera colour has changed recently then you need to see a doctor.
Otherwise if you genuinely have a healthy diet (not to high in protien and fat, high in fibre, lots of fruit and veg, lots of fluids), don't smoke or drink, exercise, sleep properly and are not taking any drugs (prescription or otherwise) you are stuck with it