It's always a good idea to keep leftover paint on hand in case of scratches and scuffs. If, however, you're holding on to old paint that no longer matches anything on your walls, or that's too dry and lumpy to use, it's time to toss it. Resist the urge to stack old paint cans at the curb with the regular trash. Paint contains toxic chemicals and must be disposed of at proper facilities. Contact your local sanitation department for disposal information.