Editing Armor Textures (PS Gimp)
Monster Hunter World's textures are all in .tex format. Photoshop and Gimp can't open .tex files. To edit the texture using Photoshop or Gimp you'll need to convert the .tex file into a .dds file so the programs can read it. Additionally you'll need plugins for Photoshop and Gimp to be able to read the .dds files.
First, you'll need a .tex converter to convert files to .dds. A good converter is "A Better MHW Texture Converter" By JodoZT.
]After downloading, put MHWTexConverter_by_Jodo.exe in a folder of your choice. It is recommended to be put in a place that is easily accessible and not your MHW directory. This where you generally want to keep all your files you have edited or plan on editing so keeping it away from the MHW directory will make it easier to organize. In addition MHW may be on an SSD and putting editing files in an SSD may not be the best usage of your SSD's space.
]In this example, we want to edit Kulve Taroth α helm. First drag the helm texture file, f_helm049_0000_BML.tex to MHWTexConverter_by_Jodo.exe. The converter will create a copy of the file and change it to .dds and be placed in the same folder. The original .tex file will not be changed and the new file DXT1_f_helm049_0000.BML.dds is the file you want to open to edit the textures.
When you are done, you will need to reverse the process and put the .dds into the texture converter to make it into a .tex that Monster Hunter World can recognize.
Notice that if any alpha pixel has been shown in your layers, the dds editing tools may not save your extra alpha channel correctly. Remember to preview the alpha channel of your texture before saving if the texture needs alpha channel.
Get the .dds plugin for Gimp here. Drop the plugin into your GIMP plugins directory in C:\Program Files\GIMP 2\lib\gimp\2.0\plug-ins.
Open the selected texture .dds file in GIMP
Open the file, do not load pre-existing mipmaps, edit the texture, export as .dds with the compression settings as DXT1 and auto generate mipmaps.