Modern balls can become dirty with use – chalk dust, oils from the hands and general use can all lead to them losing their shine.
For most modern balls, you can use a very mild soap and warm water to clean them. Definitely do not use a dish washer unless you prefer all your snooker balls white or faded, hot water should be avoided too. The best way to clean them is to use a proprietary snooker ball cleaner, this puts the shine back in them. You can also use car T-Cut followed by car wax polish. Solvents or glass cleaners should never be used.
Lastly, it’s advised you replace a set of balls every 10 years if in regular use, as it may surprise you to know they wear at different rates. The white and black end up slightly smaller (and I do mean very slightly) as they get more use than the other balls – well the white certainly does , the black depends how good you are at snooker !!! However as most of the club’s I visit seem to think the balls last forever I wouldn’t let it worry you too much :)!