This is obtainable in the area with the church (part of the your natural progression) and it's super simple to get. Now It's time to channel your inner Rambo and just stand in the mud!
The center of this area seems to have a mud moat surrounding a bit of the land so walk into it and make your way across the left side so you can encounter some enemies by the shipping containers and eventually above you by the small wooden bridge.
Besides the color, the main difference between water and mud is your movement speed so it's safe to start firing off when you notice you're walking super slow.