Once I'm done I'll move this to the DoW Skinning forum, what with it being a skinning question
The texture tool is simple enough for editing WTPs - run it, click "Extract WTP", find the WTP, select the WTP (or WTPs), click "open", wait a second and then open in your favourite paint package that can open 32-bit TGA files. Once you've finished editing, go back to the texture tool, click "Compile WTP", find one of the images from the WTP (it should remember the folder if you don't close it down), select a file from the WTP, click "open", wait a second and you've got a modified WTP.
To make monotone Guard fatigues then all you'll need to do is cut the fatigue areas from the Secondary layer (and/or any other layer they might be on) and paste them on to the Primary layer (which is the main colour, I think).
If you don't have the base WTPs then check out the
SGA Explorer and extract them from the WXP-Shared_Textures.sga file in the WXP folder.
If you can't do it yourself then stick it in the "Texture Requests" thread, or ask a few more questions about what you're having problems with