Latex Based Paint: DRIED latex based paint goes directly into your trash.  Kitty litter helps speed drying or "waste paint hardener".  After paint has dried, place can into a plastic bags before discarding in the trash.  

Oil Based Paint: Is Hazardous Waste.  Lexington Minuteman Hazardous Products Regional Facility: 
(subject to a non-Lexington resident fee) Saturday's 9 AM - 2PM. Click Here for their Schedule