Evaporate it… Pour it into an Evaporative Bucket. (this tip came from Kevin Tobin of Golden Paints).
Take a bucket and put some sand in there. Put it in your garage, on your back porch.. or under a low hanging eve. You want to avoid rain getting into your bucket. When you pour your dirty water in, the sand traps the paint and the water evaporates. Good for the environment and easy as can be. (no washing the sink after you’ve poured the paint water in)