If you are unsure about the paint composition, read the label and contact the manufacturer for recommendations on disposal. Look for a toll free number on the label. Some people don't know what to do with their leftover paint. Prevention of Household Hazardous Waste Consumers no longer need to buy extra paint thinking they won't be able to match the colors. New color and computer technology allows for more accurate color matching.
Machines can read a paint chip sample and match the color.
Buy only what is needed and use all the paint purchased. Even though larger quantities may be cheaper per gallon, difficulty with storage and total use or disposal may arise. Measure carefully the area to be painted to estimate the amount needed. Buy the least hazardous product and buy the product for the specific job and materials. Use it up, give it away to friends or donate it to charity.