If you have old latex (water-based) paint, consider drying it out or solidifying it rather than bringing it to the Boulder County Household Hazardous Waste Facility. 

According to Hilary Collins of the Boulder County Resource Conservation Division, "Dried-out latex paint is not hazardous and can be safely disposed of in a landfill. Residents can save themselves a trip to the HHW Facility or collection event by drying or solidifying their latex paint waste themselves."

Resource Conservation suggests that this be done by lining a cardboard box with a plastic bag, adding kitty litter, then pouring in the paint. Stir to coat. When the paint is dry, throw the paint-litter mixture away in the plastic bag.

Paint can also be dried out in the can, then thrown away your regular trash. If the paint is dried out outside the can, the empty can can be recycled with scrap metal.