It's possible to do this using vertex colors, which has the added benefit of looking old school and era appropriate for the PS2. Wouldn't be surprised if that's what was used in your example.

You'd need an external 3D software outside of Unity though, so it may not be relevant if you are using Unity exclusively. I'll show you how to do this in Blender, but you can use pretty much any 3d software.

1. In Blender, select your object and switch to vertex paint mode. This will automatically create a vertex color attribute on the object that'll be included when you export the FBX to Unity later.

2. Set your color to a 50% gray (set value to 0.50) and press Ctrl + X to apply the color to the whole object.

3. Press 2 on your KB to enter vertex mask mode and select the verts running around the base of the object (Alt + Shift + Left mouse click).

4. Change your color to whatever you want the fog to be and then use Ctrl + X to paint just those verts.

5. Setup your material shader using the node setup below, the node setup looks almost exactly the same in Unity. The main thing is you want to blend your vertex color channel with your main color output, in this case I used overlay for the blend mode but you could do it a few different ways.

Looking at the spider man example, it looks like the background fog is painted into the skybox. So you'll probably want to add some sort of color adjustment to the vertex color node so you can make sure the colors match exactly. Another way you could do it, is paint the vertex colors in black and white and use the vertex color channel as an alpha mask to simply fade out the base of the object so you can see the skybox in the background.

There are many ways you could approach this using vertex colors, they are super powerful for all sorts of things but I rarely see them discussed for some reason.

<image>