Colorado offers a program called PaintCare to make recycling old, unwanted paint convenient and free.
If you don't want to haul your old paint to one of the 159 drop-off sites in Colorado, you can mix it with inexpensive kitty litter, let it dry completely and dispose of it in your regular trash. If you choose to do this, leave the lids off the paint containers so the pick-up crew can see that the paint is dry.
Schools may be glad to have your old paint for theater or other projects. Be sure to call first!