They are salt ponds. Cargill Salt owns the land. They open the levee, and allow salt water to fill the pond. Then the levee is closed, and the water allowed to evaporate. The colors come from varying types and concentrations of algae, which can thrive in the ever saltier waters. Eventually the water is gone, and large equipment comes in to simply scrape the salt up off of the ground. Then the process is started over. The different ponds are different colors because the process is staggered over different ponds. Here’s a link: