From Mods Repository Wiki
It is very easy to change the appearance or color of a texture of a model. Even, with only some knowledge of Photoshop or another editing program, we can completely change the appearance (for example, by changing the type of sheet)
For first, we need to know that texture is what we have to be modified. We can do this in the Asset Viewer
Or with the help of the great Tom Xmodel Utills. With the Asset Viewer we can see the materials the model has, and from here, to search for the exact name of the texture. With the Tom Xmodel Utills we can see the names of the iwi files.
I recommend having the entire contents of the unzipped files IWD in another folder, so you can look for the best texture that we are interested in.
Second Step: we convert the iwi file that we are going to change into a dds file, for this could be easier to use the iwi2dds.exe tool
Third Step: open the dds file with the image editing program you use. If you are using Photoshop, as me, you'll need the appropiate NVIDIA dds pluguin for the version you have. http://developer.nvidia.com/nvidia-texture-tools-adobe-photoshop
Once installed the plugin, we proceed to manipulate the texture.
I recommend not tend toward the pure green color, because in nature, the pure green does not exist, and our texture would look more a plastic artificial plant than something natural. It would be unreal.
We can also make a second adjustment to try to match another tone, this always to your liking. Sometimes a slight difference in tone from the original can offer you a great result.
Fourth Step: It should be borne in mind that, when converting it previously of iwi to dds, the converter always adds "iwi_out" at the end of the name, and we must not keep it once carried out the work.
We save the file and select the format options dds as DXT1 (1 bit alpha) or better DXT5 (interpolated alpha) -I prefer this last one (*)-
Finally, we can only convert the image DDS to iwi with dds2iwi.exe, and include it within our IWD file. Nothing else is required!
(*) These formats are only used when there's an alpha channel in the image. If this is an image without alpha channel, select DXT1 (not alpha)