Recently environmental pollution has been (rightly) getting a lot of attention and people are beginning to realise the impact we are having on our planet - so if you want to learn how to do your bit when it comes to your used modeling paints read on.
Acrylic vs Solvent Based "Enamel"
The first thing to consider is what your paint is made up of. While all types of paint can contain harmful pollutants, enamel paint is almost always worse due to the organic solvents used. While the term "enamel" actually refers to the final sheen, most paint marketed as Enamel will contain organic solvent, this is the "thinner" that keeps it liquid and is a nightmare for the environment if released into the water table. Acrylic paint, on the other hand, is generally thought to have a lower environmental impact and this is one of the reasons it has grown so much in popularity in recent years. Even acrylic, however, contains oil-based compounds (acrylic, after all, is a plastic) so care still needs to be taken when disposing of old paint or cleaning out your palette.
Let It Dry
When it comes to groundwater and the environment we really can't be too careful. As a general rule, don't wash paint of any kind down the drain in your house, garden or neighbour's driveway. The solution is to let the paint dry and then dispose of as solid waste. Wipe off as much as you can from the pallete/pot with a paper towell or old newspaper - then dispose of this in the regular trash. This will reduce the amount of paint that comes off when you eventually have to rinse with water. Even the water itself can be left out to dry in the air if practical, leaving the paint behind as a solid.
Another method that can work to dry out paint and allow you to dispose of it as solid waste is cat litter. This material is made from clay (or modern clay-like alternatives) and is incredibly good at absorbing liquids, including the environmentally harmful liquids that can be in paint. If you make dioramas for your models - you may even have some lying around for rocky scenery. Just mix some litter in with the old paint or rinse water until dry and then dispose of as solid waste.
Just Keep It!
Lastly, you should always question whether you even need to dispose of your paint. Kept airtight, paint can last a very long time. Paint you've mixed can be stored in an airtight container such as a contact lens case or film cannister. Either way think carefully before you decide to throw it out in the first place!