For the sake of others who might come across this thread as well, I followed Bagginsbill's method for tattoos here:

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2716576&page=1

If you've got a transparency and a lip map that are already correctly located -- which I did -- you don't need the "3 math channels" which help to move the image map and the transparency together to the right place. On my screenshot, that's the jumble of pink to the right that I'm not bothering to show. If you need it, you can see it in BB's examples anyway.

What I did:

The EZSkin nodes are a bit different from BB's pics. But not that different.

If you select the lips material, and locate the color map, it's connected to an hsv node and a comp node. On my setup, the wires are green.

Disconnect the image map (called "Color Map") from these two nodes and connect a blender node instead.

Then connect the Blending "plug" (I don't know the correct term, but it looks like a plug to me) of the blender node to your transparency and make the background black. Set image_mapped to None so it doesn't tile (I guess if it's the same size as the original map, that's not an issue.)

Connect Input_1 of the blender node to your main color map (probably a face map -- whatever was originally there).

Connect Input_2 to your new color map (the new lip color), and set image_mapped to None so it doesn't tile.

The Blending "plug" connected to your transparency map defaults to 0.5000, to give a 50% blend, but if you want the new color map to have a stronger effect, just make that number higher. I put it to .8 and could see a new lip color very well.

Edit: The edges of the lips don't look too great because the transparency I used was a quick hack job based on a color map, and my brushwork with a mouse is terrible. :D

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.