As an example I took the black cow and made it violet.

Procedure:

• Start up World Builder
  
  • Since we only want to change a skin, we do not have to load a map.
  • Instead, we open the asset browser with F11.

• Create a new project
  
  • Inside the Asset Browser click the bottom left button (Tooltip: Create new project).
  • Another window automatically opens. Enter a name for our project (such as cow_violet).
  • The new project automatically contains an asset package called content. All modified and new data goes inside this one.
  • Our new project is written italic. This means it is not yet running (=active).

• Now we must find the texture inside the em5 project. There are different ways to do this and all of them are a little tricky. I describe just one.
  
  • Open the Prefab Browser and type cow in the searchbar.
  • Choose the prefab em5/prefab/animal/cow_black and place it into the map.
  • Open the material browser (M key). Blick the bottom left button called Pick. The mouse cursor changes to a hand.
  • With the hand cursor click the cow inside your viewport to pick its material.
  • Now the material browser displays the cows materials properties in the right table.
  • We want to change the color map. Therefore, rightclick ColorMap > Copy Global Asset ID.

• Next we want to use the global asset ID to find our texture in the asset browser.
  • In the asset browser select the asset package content in the em5 project (not the one in your project).
  • Paste the global asset ID (81654) into the search bar.
  • We find 2 textures. The simple name (cow_black_c) is a reference to the photoshop file used by the developers. We can not access this one.
  • The RIGHT texture to choose for us is the one with the long cryptic name:
    em5/texture/_derived/_material_0_crgb_aa_81654_1_18446744073709551615_1_18446744073709551615_1.
  • The reason all em5 textures have cryptic names is they were procedurally compressed during development.
  • By the way: crgb_aa means the color map is in the channels Red Green and Blue while the alpha is inside the Alpha channel

• Copy the texture to our mod project
  
  • Rightclick em5/texture/_derived/_material_0_crgb_aa_81654_1_18446744073709551615_1_18446744073709551615_1 > Copy to Asset Package > "my mod" > content.
  • Now look inside windows explorer under \EM5\data\cow_violet\content\texture\_derived to find the texture.

• Changing the texture
  • EMERGENCY 5 textures are .dds-files. To open them with Photoshop you will need NVIDIA Texture Tools for Adobe Photoshop
  • I do not explain how to use Photoshop here. There are lots of tutorials on the internet.
  • To find out the files format and mipmap settings you can use Windows Texture Viewer.
  • In this case we need to save as DXT1 with MipMaps All.
  • (Of course you can use Gimp or another program instead of Photoshop)

• Activate, play and export the mod
  
  • After changing the texture, inside the asset browser rightclick your project (not oyur asset package) and turn Running Project on
  • The change can directly be seen. Your cow should now be violet.
  • To hand your skin to other players rightclick your project again > Publish Mod Project.
  • Save a .zip-file.
  • Another player needs only drag this file into the modification window inside the launcher to load the map.