You must have Skin Manager or a similar tool to replace the texture of your Locomotive.

1) Follow the instructions on Skin Manager Mod in order to install the mod.
2) Once done, navigate to steamapps\common\Derail Valley\Mods\SkinManagerMod\Skins\loco_621
3) Drag and drop the folder included in the .zip file into this folder
4) Start the game and spawn in your loco!

NOTE: The Skin Manager Mod will randomly select the locomotive's texture. You will need to keep respawning your loco until the desired paint is randomly selected.