You can now return old paint to many paint stores! California has adopted a recycling program where they collect a small recycling fee when you buy paint….this is much like we are used to when we buy beverage containers. Most chain paint stores will accept old paint in closed, non-leaking, labeled containers. They will take both oil and water based paints. If the container is leaking or the label is gone then purchase a paint hardener at the paint store and mix into the water based paint and in a few hours it will be hard and you can throw in the regular trash. If you have oil paint in an unlabeled can you can pour it into another oil based can with a label on it for recycling.
It is wise to keep old paint cans that were used on the current paint scheme of your house as it is very helpful to a contractor like me to see what was actually used and the formula of colorants…even if the paint in the can is no longer useful.