Basic guide to getting and altering a texture:
Open Texmod, select "logging mode". Use Texmod to load ME. Use numeric keypad to go through textures until you find the one you want. Hit whatever key you told Texmod to use to save that texture. Close ME and Texmod. Open texture file (I use GIMP) and mess with it how you want. Open Texmod again, and use "package build" to make a TPF file. Then, change Texmod to "Package Mode" to load ME using TPF files you selected.
A big chunk of getting Texmod to do what you want, is manually editing Texmod's log file. It's very simple and obvious----the first 10 digits or so of each row is the memory address of a texture. The end is the name of the file you want Texmod to use to replace that texture with.
Example:
0x422AAA0D|C: blah\ blah\ blah \\MASSEFFECT.EXE_0x422AAA0D.dds
That's Ash's face, by default. Now, you could change her face file (blue lipstick?) and save it as 0x422AAA0D_blue.dds or something. Then change that line in the log file to
"0x422AAA0D|C: blah\ blah\ blah\ \MASSEFFECT.EXE_0x422AAA0D_blue.dds"
Then make a TPF file via Texmod's package build. Then, every time you use the TPF file made with that instance of the log file, it will override memory address 0x422AAA0D with the file you made, instead of the file it gets from ME.
PS--please get an existing TPF file and load it with Texmod just to see how things work---that'll explain half of what Texmod is/does. Here's a link to my "purple F Med Phoenix" armor:
Texmod will not ask you much when it makes TPF files---it just reads the lines in the Texmod.log file and does what the log file says at that moment, immediately. The log file records every texture/memory address every time you use it in logging mode, so be sure to check before you make a TPF file that the log file has exactly what you want before you do it. I keep my own file with a list of textures I edit, to cut and paste into Texmod's log as needed.