I don't think I've seen this mentioned before in the thread, but I figured I'd give a guide to the fourth type of texture file floating around in the assets, the [mech]_[part]_rgb. Unless you want a brightly coloured rainbow 'Mech, they aren't much use for direct colour, so what are they for?
Well, the rgb files are the key to the mechlab's camospec system. You select colour 1, it gets mapped to the red channel, you pick colour 2, it gets mapped to green, and so on (note: I haven't actually checked to see which colours map to which channels, but that's the basic principle). As a retexturer, you can leverage that to your benefit, since the channels aren't just on or off, but represent transparency as well. The brighter the colour in the rgb map, the more opaque the camospec. Which means if you disregard the differences in channels and just look at it as a greyscale map, it represents which parts of the texture map should be painted (the hull casing, for instance) and which should be left alone (knee joints).
With proper processing, therefore, it makes the perfect mask for any colours you want to apply to the dif map.
The first step is to desaturate the image. For Gimp, click Colors=>Desaturate, then use the "Lightness" option. For Photoshop, click Image=>Adjustments=>Desaturate.
This will give you a greyscale image, but since the lightest colours, where red=255 for instance, still aren't as light as white would be, where red, green and blue=255. The lightest point on our greyscale map isn't white, so we'll need to adjust the channel curve. In Gimp, click Colors=>Curves. In Photoshop, click Image=>Adjustments=>Curves. You'll see a little graph with a straight line from bottom left to top right, and a number of grey peaks. Grab the top right point, and move it back to the left until it's above the rightmost peak. Be aware, there's probably a big peak, followed by some tinier ones that are harder to spot. Gimp helps here by making the bottom axis black where there is data, and grey where there's nothing.
(Top: Gimp's curve editor; bottom: Photoshop's curve editor. Both adjusting the catapult_body_rgb file.)
So, now you should have an image that goes from black to white representing how much you want that area coloured. But how to make that into a mask? Well, Gimp and Photoshop do this differently, and Gimp users, you'll be pleased (and possibly surprised) that Gimp is the easier of the two.
Making a mask in Gimp:
Right click the layer, click "Add Layer Mask" then select "Grayscale copy of layer". Done.
Okay, okay, you'll need to move it to a different layer for painting as well (Photoshop actually does this bit better, but the rest of it so much worse). Easiest way to do this is to right click the layer, select "Mask to selection", right click your painting layer, click "Add Layer Mask" again, and this time select "Selection". Now we're done.
Making a mask in Photoshop:
This is a bit more tricky, as we'll be diving down into some more technical stuff. Make sure you've got just the greyscale rgb layer visible, then click the Channels tab next to the Layers tab. Right click one of the channels there (Blue will do; but Red and Green should be identical) and duplicate it. Name the new channel "Alpha".
Click Select=>Load Selection. Make sure it's set to the document you're working in, and the channel should be set to "Alpha" (assuming you named it that). Hit okay. Now select your painting layer, and click the "Add Layer Mask" button on the bottom of the Layer tab. You should now have a layer that is automatically set to your selection set. Upside, now that you've got that mask in place, you can drag it between layers, or copy it by holding alt and dragging.
And there you have it. Hope that's proven useful to you all. I think that we should be able to use the rgb map data to make templates similar to the ones Ironhawk and Hayden made for the Repainted Concept Art thread, but I haven't explored that yet.