Colour lookup tables with UVs
The characters in Off Grid use a common material which switches out Colour Lookup Tables (LUTs) to change the colour of the Character's UVs. A LUT is a colour grid, each tile in the grid corresponds to a part of the Character's body.
Each LUT is a 64x64 pixel .png texture. Each tile in that texture is 8x8 pixels.
Colors in the textures
The main texture is an Albedo colour map.
Most characters share a lot of tile positions. The First Row of all characters is the same, it has the tiles for head, eyes, hair and facial hair.
The Second Row is for Torso
The Third Row is for Arms
The Fourth Row is for Legs
The Fifth Row is for Shoes
The Final 3 Rows are common across all characters and contain colours for hats and props.
Metallic + Smoothness
The second texture is a combined Metallic + Smoothness texture. The colour of the tile sets the smoothness of the material, with black for rough & diffuse, and white for smooth and reflective surface. Then the alpha value sets the smoothness, with 0 as metallic, and 1 as non-metallic.
Only the colour texture is absolutely required, if the metallic+smoothness texture is left out the Character will be rendered with a simple diffuse material which will look fine in the game.
For more information on how Unity uses Metallic and Smoothness:
Setting up the UVs
Each part of the model is UV'd to a single color tile
Because Unity will compress the textures and that can possibly cause some issues like colour bleed around tile borders, it's best to leave a bit of extra space there. As we don't really need to cover any area of the texture in the UVs, just collapsing everything into single points and making sure that's more or less at the centre of a colour tile should work fine.
Many of these LUTs will work between all characters but to make sure the outcome is as expected use these guides when making new LUTs or altering existing LUTs.