The main cause of a discolored sclera is dehydration and fatigue. So make sure you drink enough fluids and get a good night’s sleep, every night. Also, vitamin E is a great antioxidant to help you flush your body of toxins.
Alternatively, you could use eye drops which contain a vascular constrictor. This constricts all the blood vessels in your eyes, bringing out the white more. Unfortunately, if you wear contact lenses you cannot use these drops. And only use these drops on SPECIAL occasions as they should not be used often at all.
A make-up trick you could try is using black or white eyeliner instead of black, this should make your eyes appear brighter.