Parts Craft DLC first screenshot

Took a stab at this:

 

Sure. This is how I was doing it when I was using the mod suite back when, (this assumes you're using photoshop)...

1) In photoshop, open your completed custom part png.
2) Add a new layer, and position it beneath the the layer with the custom part.
3) Fill the new layer with black.
4) Go back to the custom part layer.
5) Use the magic wand tool and select a pixel of the brightest white. (I use "point sample" and tolerance of "0" generally.)
6) Once you've selected a white area, then right click and select "similar." Now you've selected every bright white area within the custom part png image.
7) Right click again and "select inverse," so that now you've selected every area of the part png that ISN'T brightest white.
8) While all of the non-bright-white areas are still selected, go to the top ribbon and choose "select-->modify-->feather." I find a radius of "3" gives the right result.
9) Go again to the top ribbon and select "edit-->clear." Now you've erased everything from the custom part png except the bight white areas, with a nice soft feathered edge around them.
10) Save is as a new png, using the same name as you would use for the part, but with adding "_h" at the end to denote it as a highlight, (ie. "m_bo_Vest Stripes_m_1_h" for example.) Make sure both layers are turned on, so in the saved png, the background appears black, as opposed to clear like with a regular part png.
11) When doing the tex file creation in unity, add both the original part png and the highlight png as assets and follow the rest of the process as normal. You don't have to add anything extra to the partlist text file beyond what Carl's instructions call for.

That's it. And it creates a highlight that's very close to what the default parts use.

A few additional notes...
If your original part png doesn't have much of a bright white area, you can try selecting the lightest gray and using a tolerance of 10-20 to get it to ultimately select the brightest two shades, (pure white and lightest gray,) when selecting "similar," which will give you a larger highlight area. And/or when you get to the feathering step, you can also try a radius of "2" as opposed to "3" will also leave a larger highlight area visible after clearing the rest. I've had to do either or both a few times with smaller parts like the boot trims and pieces.

It sounds like a lot of steps, but it honestly doesn't take more than a minute or two if done while you're creating the parts png themselves. Even if you don't use the highlights, (and I generally keep them to 25% unless I'm doing shiny boots,) it really does make a difference in how they look in game and how well they match the default parts, especially when colored black.