In this recipe, everything will be created from scratch, so you don't need to prepare any special assets. You can also use the example project and go to the Chapter 07 Special effectsRecipe 03 Using the animation window to animate public script variables directory. You can find the Example.unity scene there. If you open it and press the space bar, you can observe the fog changing color and density.

It is achieved by animating the public variables of a script.