Excessive alcohol can alter the brightness of you eyes sometimes to extremes in cases of cirrhosis. Stay off the booze for a few days and I am sure you will notice them brighten. A quick fix method are eye drops 'Eye Dew' I think it is called but I would not recommend this other than for very occasional use. I have also noticed that as you get older the whiteness of your eyes does change, I am not suggesting that you have aged overnight but maybe you have only recently noticed a change which may have been ongoing for a while.
If you dont drink for a while and they dont brighten then you may wish to consult your doctor.