These posts has helped me alot. First, I didn't make the Multi channel mixer to work as illustrated in former posts above. I was using Don, and he don't have separate eyelashes grouped. I then only got visible lashes.

Then I saw the trans map didn't include the rest of the color map, so you can see how it is included by painting white around the lashes.

Just showing and telling so I can contribute with some more in this subject, because I found it quite hard to grasp compared to other apps.