Amazon Environmental is America's leading recycler of leftover water based (latex) paint. Paint is recycled back into paint.
This is done through a process of first sorting for colours and filtering. Paints of the same colours are then pumped into a mixing tank and mixed. The blend is tested, and additives added to get it to a consistent blend.
The final product meets the same specifications as virgin paint, and is unconditionally guaranteed.
Unfortunately recycling of water based paint, whilst relatively straightforward, is not common.